LCOV - code coverage report
Current view: top level - kernel/locking - rwsem.c (source / functions) Hit Total Coverage
Test: Real Lines: 313 333 94.0 %
Date: 2020-10-17 15:46:43 Functions: 7 36 19.4 %
Legend: Neither, QEMU, Real, Both Branches: 0 0 -

           Branch data     Line data    Source code
       1                 :            : // SPDX-License-Identifier: GPL-2.0
       2                 :            : /* kernel/rwsem.c: R/W semaphores, public implementation
       3                 :            :  *
       4                 :            :  * Written by David Howells (dhowells@redhat.com).
       5                 :            :  * Derived from asm-i386/semaphore.h
       6                 :            :  *
       7                 :            :  * Writer lock-stealing by Alex Shi <alex.shi@intel.com>
       8                 :            :  * and Michel Lespinasse <walken@google.com>
       9                 :            :  *
      10                 :            :  * Optimistic spinning by Tim Chen <tim.c.chen@intel.com>
      11                 :            :  * and Davidlohr Bueso <davidlohr@hp.com>. Based on mutexes.
      12                 :            :  *
      13                 :            :  * Rwsem count bit fields re-definition and rwsem rearchitecture by
      14                 :            :  * Waiman Long <longman@redhat.com> and
      15                 :            :  * Peter Zijlstra <peterz@infradead.org>.
      16                 :            :  */
      17                 :            : 
      18                 :            : #include <linux/types.h>
      19                 :            : #include <linux/kernel.h>
      20                 :            : #include <linux/sched.h>
      21                 :            : #include <linux/sched/rt.h>
      22                 :            : #include <linux/sched/task.h>
      23                 :            : #include <linux/sched/debug.h>
      24                 :            : #include <linux/sched/wake_q.h>
      25                 :            : #include <linux/sched/signal.h>
      26                 :            : #include <linux/sched/clock.h>
      27                 :            : #include <linux/export.h>
      28                 :            : #include <linux/rwsem.h>
      29                 :            : #include <linux/atomic.h>
      30                 :            : 
      31                 :            : #include "rwsem.h"
      32                 :            : #include "lock_events.h"
      33                 :            : 
      34                 :            : /*
      35                 :            :  * The least significant 3 bits of the owner value has the following
      36                 :            :  * meanings when set.
      37                 :            :  *  - Bit 0: RWSEM_READER_OWNED - The rwsem is owned by readers
      38                 :            :  *  - Bit 1: RWSEM_RD_NONSPINNABLE - Readers cannot spin on this lock.
      39                 :            :  *  - Bit 2: RWSEM_WR_NONSPINNABLE - Writers cannot spin on this lock.
      40                 :            :  *
      41                 :            :  * When the rwsem is either owned by an anonymous writer, or it is
      42                 :            :  * reader-owned, but a spinning writer has timed out, both nonspinnable
      43                 :            :  * bits will be set to disable optimistic spinning by readers and writers.
      44                 :            :  * In the later case, the last unlocking reader should then check the
      45                 :            :  * writer nonspinnable bit and clear it only to give writers preference
      46                 :            :  * to acquire the lock via optimistic spinning, but not readers. Similar
      47                 :            :  * action is also done in the reader slowpath.
      48                 :            : 
      49                 :            :  * When a writer acquires a rwsem, it puts its task_struct pointer
      50                 :            :  * into the owner field. It is cleared after an unlock.
      51                 :            :  *
      52                 :            :  * When a reader acquires a rwsem, it will also puts its task_struct
      53                 :            :  * pointer into the owner field with the RWSEM_READER_OWNED bit set.
      54                 :            :  * On unlock, the owner field will largely be left untouched. So
      55                 :            :  * for a free or reader-owned rwsem, the owner value may contain
      56                 :            :  * information about the last reader that acquires the rwsem.
      57                 :            :  *
      58                 :            :  * That information may be helpful in debugging cases where the system
      59                 :            :  * seems to hang on a reader owned rwsem especially if only one reader
      60                 :            :  * is involved. Ideally we would like to track all the readers that own
      61                 :            :  * a rwsem, but the overhead is simply too big.
      62                 :            :  *
      63                 :            :  * Reader optimistic spinning is helpful when the reader critical section
      64                 :            :  * is short and there aren't that many readers around. It makes readers
      65                 :            :  * relatively more preferred than writers. When a writer times out spinning
      66                 :            :  * on a reader-owned lock and set the nospinnable bits, there are two main
      67                 :            :  * reasons for that.
      68                 :            :  *
      69                 :            :  *  1) The reader critical section is long, perhaps the task sleeps after
      70                 :            :  *     acquiring the read lock.
      71                 :            :  *  2) There are just too many readers contending the lock causing it to
      72                 :            :  *     take a while to service all of them.
      73                 :            :  *
      74                 :            :  * In the former case, long reader critical section will impede the progress
      75                 :            :  * of writers which is usually more important for system performance. In
      76                 :            :  * the later case, reader optimistic spinning tends to make the reader
      77                 :            :  * groups that contain readers that acquire the lock together smaller
      78                 :            :  * leading to more of them. That may hurt performance in some cases. In
      79                 :            :  * other words, the setting of nonspinnable bits indicates that reader
      80                 :            :  * optimistic spinning may not be helpful for those workloads that cause
      81                 :            :  * it.
      82                 :            :  *
      83                 :            :  * Therefore, any writers that had observed the setting of the writer
      84                 :            :  * nonspinnable bit for a given rwsem after they fail to acquire the lock
      85                 :            :  * via optimistic spinning will set the reader nonspinnable bit once they
      86                 :            :  * acquire the write lock. Similarly, readers that observe the setting
      87                 :            :  * of reader nonspinnable bit at slowpath entry will set the reader
      88                 :            :  * nonspinnable bits when they acquire the read lock via the wakeup path.
      89                 :            :  *
      90                 :            :  * Once the reader nonspinnable bit is on, it will only be reset when
      91                 :            :  * a writer is able to acquire the rwsem in the fast path or somehow a
      92                 :            :  * reader or writer in the slowpath doesn't observe the nonspinable bit.
      93                 :            :  *
      94                 :            :  * This is to discourage reader optmistic spinning on that particular
      95                 :            :  * rwsem and make writers more preferred. This adaptive disabling of reader
      96                 :            :  * optimistic spinning will alleviate the negative side effect of this
      97                 :            :  * feature.
      98                 :            :  */
      99                 :            : #define RWSEM_READER_OWNED      (1UL << 0)
     100                 :            : #define RWSEM_RD_NONSPINNABLE   (1UL << 1)
     101                 :            : #define RWSEM_WR_NONSPINNABLE   (1UL << 2)
     102                 :            : #define RWSEM_NONSPINNABLE      (RWSEM_RD_NONSPINNABLE | RWSEM_WR_NONSPINNABLE)
     103                 :            : #define RWSEM_OWNER_FLAGS_MASK  (RWSEM_READER_OWNED | RWSEM_NONSPINNABLE)
     104                 :            : 
     105                 :            : #ifdef CONFIG_DEBUG_RWSEMS
     106                 :            : # define DEBUG_RWSEMS_WARN_ON(c, sem)   do {                    \
     107                 :            :         if (!debug_locks_silent &&                              \
     108                 :            :             WARN_ONCE(c, "DEBUG_RWSEMS_WARN_ON(%s): count = 0x%lx, magic = 0x%lx, owner = 0x%lx, curr 0x%lx, list %sempty\n",\
     109                 :            :                 #c, atomic_long_read(&(sem)->count),             \
     110                 :            :                 (unsigned long) sem->magic,                  \
     111                 :            :                 atomic_long_read(&(sem)->owner), (long)current,  \
     112                 :            :                 list_empty(&(sem)->wait_list) ? "" : "not "))        \
     113                 :            :                         debug_locks_off();                      \
     114                 :            :         } while (0)
     115                 :            : #else
     116                 :            : # define DEBUG_RWSEMS_WARN_ON(c, sem)
     117                 :            : #endif
     118                 :            : 
     119                 :            : /*
     120                 :            :  * On 64-bit architectures, the bit definitions of the count are:
     121                 :            :  *
     122                 :            :  * Bit  0    - writer locked bit
     123                 :            :  * Bit  1    - waiters present bit
     124                 :            :  * Bit  2    - lock handoff bit
     125                 :            :  * Bits 3-7  - reserved
     126                 :            :  * Bits 8-62 - 55-bit reader count
     127                 :            :  * Bit  63   - read fail bit
     128                 :            :  *
     129                 :            :  * On 32-bit architectures, the bit definitions of the count are:
     130                 :            :  *
     131                 :            :  * Bit  0    - writer locked bit
     132                 :            :  * Bit  1    - waiters present bit
     133                 :            :  * Bit  2    - lock handoff bit
     134                 :            :  * Bits 3-7  - reserved
     135                 :            :  * Bits 8-30 - 23-bit reader count
     136                 :            :  * Bit  31   - read fail bit
     137                 :            :  *
     138                 :            :  * It is not likely that the most significant bit (read fail bit) will ever
     139                 :            :  * be set. This guard bit is still checked anyway in the down_read() fastpath
     140                 :            :  * just in case we need to use up more of the reader bits for other purpose
     141                 :            :  * in the future.
     142                 :            :  *
     143                 :            :  * atomic_long_fetch_add() is used to obtain reader lock, whereas
     144                 :            :  * atomic_long_cmpxchg() will be used to obtain writer lock.
     145                 :            :  *
     146                 :            :  * There are three places where the lock handoff bit may be set or cleared.
     147                 :            :  * 1) rwsem_mark_wake() for readers.
     148                 :            :  * 2) rwsem_try_write_lock() for writers.
     149                 :            :  * 3) Error path of rwsem_down_write_slowpath().
     150                 :            :  *
     151                 :            :  * For all the above cases, wait_lock will be held. A writer must also
     152                 :            :  * be the first one in the wait_list to be eligible for setting the handoff
     153                 :            :  * bit. So concurrent setting/clearing of handoff bit is not possible.
     154                 :            :  */
     155                 :            : #define RWSEM_WRITER_LOCKED     (1UL << 0)
     156                 :            : #define RWSEM_FLAG_WAITERS      (1UL << 1)
     157                 :            : #define RWSEM_FLAG_HANDOFF      (1UL << 2)
     158                 :            : #define RWSEM_FLAG_READFAIL     (1UL << (BITS_PER_LONG - 1))
     159                 :            : 
     160                 :            : #define RWSEM_READER_SHIFT      8
     161                 :            : #define RWSEM_READER_BIAS       (1UL << RWSEM_READER_SHIFT)
     162                 :            : #define RWSEM_READER_MASK       (~(RWSEM_READER_BIAS - 1))
     163                 :            : #define RWSEM_WRITER_MASK       RWSEM_WRITER_LOCKED
     164                 :            : #define RWSEM_LOCK_MASK         (RWSEM_WRITER_MASK|RWSEM_READER_MASK)
     165                 :            : #define RWSEM_READ_FAILED_MASK  (RWSEM_WRITER_MASK|RWSEM_FLAG_WAITERS|\
     166                 :            :                                  RWSEM_FLAG_HANDOFF|RWSEM_FLAG_READFAIL)
     167                 :            : 
     168                 :            : /*
     169                 :            :  * All writes to owner are protected by WRITE_ONCE() to make sure that
     170                 :            :  * store tearing can't happen as optimistic spinners may read and use
     171                 :            :  * the owner value concurrently without lock. Read from owner, however,
     172                 :            :  * may not need READ_ONCE() as long as the pointer value is only used
     173                 :            :  * for comparison and isn't being dereferenced.
     174                 :            :  */
     175                 :            : static inline void rwsem_set_owner(struct rw_semaphore *sem)
     176                 :            : {
     177                 :          3 :         atomic_long_set(&sem->owner, (long)current);
     178                 :            : }
     179                 :            : 
     180                 :            : static inline void rwsem_clear_owner(struct rw_semaphore *sem)
     181                 :            : {
     182                 :            :         atomic_long_set(&sem->owner, 0);
     183                 :            : }
     184                 :            : 
     185                 :            : /*
     186                 :            :  * Test the flags in the owner field.
     187                 :            :  */
     188                 :            : static inline bool rwsem_test_oflags(struct rw_semaphore *sem, long flags)
     189                 :            : {
     190                 :          3 :         return atomic_long_read(&sem->owner) & flags;
     191                 :            : }
     192                 :            : 
     193                 :            : /*
     194                 :            :  * The task_struct pointer of the last owning reader will be left in
     195                 :            :  * the owner field.
     196                 :            :  *
     197                 :            :  * Note that the owner value just indicates the task has owned the rwsem
     198                 :            :  * previously, it may not be the real owner or one of the real owners
     199                 :            :  * anymore when that field is examined, so take it with a grain of salt.
     200                 :            :  *
     201                 :            :  * The reader non-spinnable bit is preserved.
     202                 :            :  */
     203                 :            : static inline void __rwsem_set_reader_owned(struct rw_semaphore *sem,
     204                 :            :                                             struct task_struct *owner)
     205                 :            : {
     206                 :          3 :         unsigned long val = (unsigned long)owner | RWSEM_READER_OWNED |
     207                 :          3 :                 (atomic_long_read(&sem->owner) & RWSEM_RD_NONSPINNABLE);
     208                 :            : 
     209                 :            :         atomic_long_set(&sem->owner, val);
     210                 :            : }
     211                 :            : 
     212                 :            : static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
     213                 :            : {
     214                 :          3 :         __rwsem_set_reader_owned(sem, current);
     215                 :            : }
     216                 :            : 
     217                 :            : /*
     218                 :            :  * Return true if the rwsem is owned by a reader.
     219                 :            :  */
     220                 :            : static inline bool is_rwsem_reader_owned(struct rw_semaphore *sem)
     221                 :            : {
     222                 :            : #ifdef CONFIG_DEBUG_RWSEMS
     223                 :            :         /*
     224                 :            :          * Check the count to see if it is write-locked.
     225                 :            :          */
     226                 :            :         long count = atomic_long_read(&sem->count);
     227                 :            : 
     228                 :            :         if (count & RWSEM_WRITER_MASK)
     229                 :            :                 return false;
     230                 :            : #endif
     231                 :            :         return rwsem_test_oflags(sem, RWSEM_READER_OWNED);
     232                 :            : }
     233                 :            : 
     234                 :            : #ifdef CONFIG_DEBUG_RWSEMS
     235                 :            : /*
     236                 :            :  * With CONFIG_DEBUG_RWSEMS configured, it will make sure that if there
     237                 :            :  * is a task pointer in owner of a reader-owned rwsem, it will be the
     238                 :            :  * real owner or one of the real owners. The only exception is when the
     239                 :            :  * unlock is done by up_read_non_owner().
     240                 :            :  */
     241                 :            : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
     242                 :            : {
     243                 :            :         unsigned long val = atomic_long_read(&sem->owner);
     244                 :            : 
     245                 :            :         while ((val & ~RWSEM_OWNER_FLAGS_MASK) == (unsigned long)current) {
     246                 :            :                 if (atomic_long_try_cmpxchg(&sem->owner, &val,
     247                 :            :                                             val & RWSEM_OWNER_FLAGS_MASK))
     248                 :            :                         return;
     249                 :            :         }
     250                 :            : }
     251                 :            : #else
     252                 :            : static inline void rwsem_clear_reader_owned(struct rw_semaphore *sem)
     253                 :            : {
     254                 :            : }
     255                 :            : #endif
     256                 :            : 
     257                 :            : /*
     258                 :            :  * Set the RWSEM_NONSPINNABLE bits if the RWSEM_READER_OWNED flag
     259                 :            :  * remains set. Otherwise, the operation will be aborted.
     260                 :            :  */
     261                 :          3 : static inline void rwsem_set_nonspinnable(struct rw_semaphore *sem)
     262                 :            : {
     263                 :          3 :         unsigned long owner = atomic_long_read(&sem->owner);
     264                 :            : 
     265                 :            :         do {
     266                 :          3 :                 if (!(owner & RWSEM_READER_OWNED))
     267                 :            :                         break;
     268                 :          3 :                 if (owner & RWSEM_NONSPINNABLE)
     269                 :            :                         break;
     270                 :          3 :         } while (!atomic_long_try_cmpxchg(&sem->owner, &owner,
     271                 :          3 :                                           owner | RWSEM_NONSPINNABLE));
     272                 :          3 : }
     273                 :            : 
     274                 :          3 : static inline bool rwsem_read_trylock(struct rw_semaphore *sem)
     275                 :            : {
     276                 :          3 :         long cnt = atomic_long_add_return_acquire(RWSEM_READER_BIAS, &sem->count);
     277                 :          3 :         if (WARN_ON_ONCE(cnt < 0))
     278                 :          0 :                 rwsem_set_nonspinnable(sem);
     279                 :          3 :         return !(cnt & RWSEM_READ_FAILED_MASK);
     280                 :            : }
     281                 :            : 
     282                 :            : /*
     283                 :            :  * Return just the real task structure pointer of the owner
     284                 :            :  */
     285                 :            : static inline struct task_struct *rwsem_owner(struct rw_semaphore *sem)
     286                 :            : {
     287                 :            :         return (struct task_struct *)
     288                 :            :                 (atomic_long_read(&sem->owner) & ~RWSEM_OWNER_FLAGS_MASK);
     289                 :            : }
     290                 :            : 
     291                 :            : /*
     292                 :            :  * Return the real task structure pointer of the owner and the embedded
     293                 :            :  * flags in the owner. pflags must be non-NULL.
     294                 :            :  */
     295                 :            : static inline struct task_struct *
     296                 :            : rwsem_owner_flags(struct rw_semaphore *sem, unsigned long *pflags)
     297                 :            : {
     298                 :            :         unsigned long owner = atomic_long_read(&sem->owner);
     299                 :            : 
     300                 :          3 :         *pflags = owner & RWSEM_OWNER_FLAGS_MASK;
     301                 :          3 :         return (struct task_struct *)(owner & ~RWSEM_OWNER_FLAGS_MASK);
     302                 :            : }
     303                 :            : 
     304                 :            : /*
     305                 :            :  * Guide to the rw_semaphore's count field.
     306                 :            :  *
     307                 :            :  * When the RWSEM_WRITER_LOCKED bit in count is set, the lock is owned
     308                 :            :  * by a writer.
     309                 :            :  *
     310                 :            :  * The lock is owned by readers when
     311                 :            :  * (1) the RWSEM_WRITER_LOCKED isn't set in count,
     312                 :            :  * (2) some of the reader bits are set in count, and
     313                 :            :  * (3) the owner field has RWSEM_READ_OWNED bit set.
     314                 :            :  *
     315                 :            :  * Having some reader bits set is not enough to guarantee a readers owned
     316                 :            :  * lock as the readers may be in the process of backing out from the count
     317                 :            :  * and a writer has just released the lock. So another writer may steal
     318                 :            :  * the lock immediately after that.
     319                 :            :  */
     320                 :            : 
     321                 :            : /*
     322                 :            :  * Initialize an rwsem:
     323                 :            :  */
     324                 :          3 : void __init_rwsem(struct rw_semaphore *sem, const char *name,
     325                 :            :                   struct lock_class_key *key)
     326                 :            : {
     327                 :            : #ifdef CONFIG_DEBUG_LOCK_ALLOC
     328                 :            :         /*
     329                 :            :          * Make sure we are not reinitializing a held semaphore:
     330                 :            :          */
     331                 :            :         debug_check_no_locks_freed((void *)sem, sizeof(*sem));
     332                 :            :         lockdep_init_map(&sem->dep_map, name, key, 0);
     333                 :            : #endif
     334                 :            : #ifdef CONFIG_DEBUG_RWSEMS
     335                 :            :         sem->magic = sem;
     336                 :            : #endif
     337                 :            :         atomic_long_set(&sem->count, RWSEM_UNLOCKED_VALUE);
     338                 :          3 :         raw_spin_lock_init(&sem->wait_lock);
     339                 :          3 :         INIT_LIST_HEAD(&sem->wait_list);
     340                 :            :         atomic_long_set(&sem->owner, 0L);
     341                 :            : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
     342                 :            :         osq_lock_init(&sem->osq);
     343                 :            : #endif
     344                 :          3 : }
     345                 :            : EXPORT_SYMBOL(__init_rwsem);
     346                 :            : 
     347                 :            : enum rwsem_waiter_type {
     348                 :            :         RWSEM_WAITING_FOR_WRITE,
     349                 :            :         RWSEM_WAITING_FOR_READ
     350                 :            : };
     351                 :            : 
     352                 :            : struct rwsem_waiter {
     353                 :            :         struct list_head list;
     354                 :            :         struct task_struct *task;
     355                 :            :         enum rwsem_waiter_type type;
     356                 :            :         unsigned long timeout;
     357                 :            :         unsigned long last_rowner;
     358                 :            : };
     359                 :            : #define rwsem_first_waiter(sem) \
     360                 :            :         list_first_entry(&sem->wait_list, struct rwsem_waiter, list)
     361                 :            : 
     362                 :            : enum rwsem_wake_type {
     363                 :            :         RWSEM_WAKE_ANY,         /* Wake whatever's at head of wait list */
     364                 :            :         RWSEM_WAKE_READERS,     /* Wake readers only */
     365                 :            :         RWSEM_WAKE_READ_OWNED   /* Waker thread holds the read lock */
     366                 :            : };
     367                 :            : 
     368                 :            : enum writer_wait_state {
     369                 :            :         WRITER_NOT_FIRST,       /* Writer is not first in wait list */
     370                 :            :         WRITER_FIRST,           /* Writer is first in wait list     */
     371                 :            :         WRITER_HANDOFF          /* Writer is first & handoff needed */
     372                 :            : };
     373                 :            : 
     374                 :            : /*
     375                 :            :  * The typical HZ value is either 250 or 1000. So set the minimum waiting
     376                 :            :  * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait
     377                 :            :  * queue before initiating the handoff protocol.
     378                 :            :  */
     379                 :            : #define RWSEM_WAIT_TIMEOUT      DIV_ROUND_UP(HZ, 250)
     380                 :            : 
     381                 :            : /*
     382                 :            :  * Magic number to batch-wakeup waiting readers, even when writers are
     383                 :            :  * also present in the queue. This both limits the amount of work the
     384                 :            :  * waking thread must do and also prevents any potential counter overflow,
     385                 :            :  * however unlikely.
     386                 :            :  */
     387                 :            : #define MAX_READERS_WAKEUP      0x100
     388                 :            : 
     389                 :            : /*
     390                 :            :  * handle the lock release when processes blocked on it that can now run
     391                 :            :  * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must
     392                 :            :  *   have been set.
     393                 :            :  * - there must be someone on the queue
     394                 :            :  * - the wait_lock must be held by the caller
     395                 :            :  * - tasks are marked for wakeup, the caller must later invoke wake_up_q()
     396                 :            :  *   to actually wakeup the blocked task(s) and drop the reference count,
     397                 :            :  *   preferably when the wait_lock is released
     398                 :            :  * - woken process blocks are discarded from the list after having task zeroed
     399                 :            :  * - writers are only marked woken if downgrading is false
     400                 :            :  */
     401                 :          3 : static void rwsem_mark_wake(struct rw_semaphore *sem,
     402                 :            :                             enum rwsem_wake_type wake_type,
     403                 :            :                             struct wake_q_head *wake_q)
     404                 :            : {
     405                 :            :         struct rwsem_waiter *waiter, *tmp;
     406                 :            :         long oldcount, woken = 0, adjustment = 0;
     407                 :            :         struct list_head wlist;
     408                 :            : 
     409                 :            :         lockdep_assert_held(&sem->wait_lock);
     410                 :            : 
     411                 :            :         /*
     412                 :            :          * Take a peek at the queue head waiter such that we can determine
     413                 :            :          * the wakeup(s) to perform.
     414                 :            :          */
     415                 :          3 :         waiter = rwsem_first_waiter(sem);
     416                 :            : 
     417                 :          3 :         if (waiter->type == RWSEM_WAITING_FOR_WRITE) {
     418                 :          3 :                 if (wake_type == RWSEM_WAKE_ANY) {
     419                 :            :                         /*
     420                 :            :                          * Mark writer at the front of the queue for wakeup.
     421                 :            :                          * Until the task is actually later awoken later by
     422                 :            :                          * the caller, other writers are able to steal it.
     423                 :            :                          * Readers, on the other hand, will block as they
     424                 :            :                          * will notice the queued writer.
     425                 :            :                          */
     426                 :          3 :                         wake_q_add(wake_q, waiter->task);
     427                 :            :                         lockevent_inc(rwsem_wake_writer);
     428                 :            :                 }
     429                 :            : 
     430                 :          3 :                 return;
     431                 :            :         }
     432                 :            : 
     433                 :            :         /*
     434                 :            :          * No reader wakeup if there are too many of them already.
     435                 :            :          */
     436                 :          3 :         if (unlikely(atomic_long_read(&sem->count) < 0))
     437                 :            :                 return;
     438                 :            : 
     439                 :            :         /*
     440                 :            :          * Writers might steal the lock before we grant it to the next reader.
     441                 :            :          * We prefer to do the first reader grant before counting readers
     442                 :            :          * so we can bail out early if a writer stole the lock.
     443                 :            :          */
     444                 :          3 :         if (wake_type != RWSEM_WAKE_READ_OWNED) {
     445                 :            :                 struct task_struct *owner;
     446                 :            : 
     447                 :            :                 adjustment = RWSEM_READER_BIAS;
     448                 :          3 :                 oldcount = atomic_long_fetch_add(adjustment, &sem->count);
     449                 :          3 :                 if (unlikely(oldcount & RWSEM_WRITER_MASK)) {
     450                 :            :                         /*
     451                 :            :                          * When we've been waiting "too" long (for writers
     452                 :            :                          * to give up the lock), request a HANDOFF to
     453                 :            :                          * force the issue.
     454                 :            :                          */
     455                 :          3 :                         if (!(oldcount & RWSEM_FLAG_HANDOFF) &&
     456                 :          3 :                             time_after(jiffies, waiter->timeout)) {
     457                 :            :                                 adjustment -= RWSEM_FLAG_HANDOFF;
     458                 :            :                                 lockevent_inc(rwsem_rlock_handoff);
     459                 :            :                         }
     460                 :            : 
     461                 :          3 :                         atomic_long_add(-adjustment, &sem->count);
     462                 :            :                         return;
     463                 :            :                 }
     464                 :            :                 /*
     465                 :            :                  * Set it to reader-owned to give spinners an early
     466                 :            :                  * indication that readers now have the lock.
     467                 :            :                  * The reader nonspinnable bit seen at slowpath entry of
     468                 :            :                  * the reader is copied over.
     469                 :            :                  */
     470                 :          3 :                 owner = waiter->task;
     471                 :          3 :                 if (waiter->last_rowner & RWSEM_RD_NONSPINNABLE) {
     472                 :          3 :                         owner = (void *)((unsigned long)owner | RWSEM_RD_NONSPINNABLE);
     473                 :            :                         lockevent_inc(rwsem_opt_norspin);
     474                 :            :                 }
     475                 :            :                 __rwsem_set_reader_owned(sem, owner);
     476                 :            :         }
     477                 :            : 
     478                 :            :         /*
     479                 :            :          * Grant up to MAX_READERS_WAKEUP read locks to all the readers in the
     480                 :            :          * queue. We know that the woken will be at least 1 as we accounted
     481                 :            :          * for above. Note we increment the 'active part' of the count by the
     482                 :            :          * number of readers before waking any processes up.
     483                 :            :          *
     484                 :            :          * This is an adaptation of the phase-fair R/W locks where at the
     485                 :            :          * reader phase (first waiter is a reader), all readers are eligible
     486                 :            :          * to acquire the lock at the same time irrespective of their order
     487                 :            :          * in the queue. The writers acquire the lock according to their
     488                 :            :          * order in the queue.
     489                 :            :          *
     490                 :            :          * We have to do wakeup in 2 passes to prevent the possibility that
     491                 :            :          * the reader count may be decremented before it is incremented. It
     492                 :            :          * is because the to-be-woken waiter may not have slept yet. So it
     493                 :            :          * may see waiter->task got cleared, finish its critical section and
     494                 :            :          * do an unlock before the reader count increment.
     495                 :            :          *
     496                 :            :          * 1) Collect the read-waiters in a separate list, count them and
     497                 :            :          *    fully increment the reader count in rwsem.
     498                 :            :          * 2) For each waiters in the new list, clear waiter->task and
     499                 :            :          *    put them into wake_q to be woken up later.
     500                 :            :          */
     501                 :            :         INIT_LIST_HEAD(&wlist);
     502                 :          3 :         list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
     503                 :          3 :                 if (waiter->type == RWSEM_WAITING_FOR_WRITE)
     504                 :          3 :                         continue;
     505                 :            : 
     506                 :          3 :                 woken++;
     507                 :            :                 list_move_tail(&waiter->list, &wlist);
     508                 :            : 
     509                 :            :                 /*
     510                 :            :                  * Limit # of readers that can be woken up per wakeup call.
     511                 :            :                  */
     512                 :          3 :                 if (woken >= MAX_READERS_WAKEUP)
     513                 :            :                         break;
     514                 :            :         }
     515                 :            : 
     516                 :          3 :         adjustment = woken * RWSEM_READER_BIAS - adjustment;
     517                 :            :         lockevent_cond_inc(rwsem_wake_reader, woken);
     518                 :          3 :         if (list_empty(&sem->wait_list)) {
     519                 :            :                 /* hit end of list above */
     520                 :          3 :                 adjustment -= RWSEM_FLAG_WAITERS;
     521                 :            :         }
     522                 :            : 
     523                 :            :         /*
     524                 :            :          * When we've woken a reader, we no longer need to force writers
     525                 :            :          * to give up the lock and we can clear HANDOFF.
     526                 :            :          */
     527                 :          3 :         if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF))
     528                 :          3 :                 adjustment -= RWSEM_FLAG_HANDOFF;
     529                 :            : 
     530                 :          3 :         if (adjustment)
     531                 :          3 :                 atomic_long_add(adjustment, &sem->count);
     532                 :            : 
     533                 :            :         /* 2nd pass */
     534                 :          3 :         list_for_each_entry_safe(waiter, tmp, &wlist, list) {
     535                 :            :                 struct task_struct *tsk;
     536                 :            : 
     537                 :          3 :                 tsk = waiter->task;
     538                 :            :                 get_task_struct(tsk);
     539                 :            : 
     540                 :            :                 /*
     541                 :            :                  * Ensure calling get_task_struct() before setting the reader
     542                 :            :                  * waiter to nil such that rwsem_down_read_slowpath() cannot
     543                 :            :                  * race with do_exit() by always holding a reference count
     544                 :            :                  * to the task to wakeup.
     545                 :            :                  */
     546                 :          3 :                 smp_store_release(&waiter->task, NULL);
     547                 :            :                 /*
     548                 :            :                  * Ensure issuing the wakeup (either by us or someone else)
     549                 :            :                  * after setting the reader waiter to nil.
     550                 :            :                  */
     551                 :          3 :                 wake_q_add_safe(wake_q, tsk);
     552                 :            :         }
     553                 :            : }
     554                 :            : 
     555                 :            : /*
     556                 :            :  * This function must be called with the sem->wait_lock held to prevent
     557                 :            :  * race conditions between checking the rwsem wait list and setting the
     558                 :            :  * sem->count accordingly.
     559                 :            :  *
     560                 :            :  * If wstate is WRITER_HANDOFF, it will make sure that either the handoff
     561                 :            :  * bit is set or the lock is acquired with handoff bit cleared.
     562                 :            :  */
     563                 :          3 : static inline bool rwsem_try_write_lock(struct rw_semaphore *sem,
     564                 :            :                                         enum writer_wait_state wstate)
     565                 :            : {
     566                 :            :         long count, new;
     567                 :            : 
     568                 :            :         lockdep_assert_held(&sem->wait_lock);
     569                 :            : 
     570                 :          3 :         count = atomic_long_read(&sem->count);
     571                 :            :         do {
     572                 :          3 :                 bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF);
     573                 :            : 
     574                 :          3 :                 if (has_handoff && wstate == WRITER_NOT_FIRST)
     575                 :            :                         return false;
     576                 :            : 
     577                 :            :                 new = count;
     578                 :            : 
     579                 :          3 :                 if (count & RWSEM_LOCK_MASK) {
     580                 :          3 :                         if (has_handoff || (wstate != WRITER_HANDOFF))
     581                 :            :                                 return false;
     582                 :            : 
     583                 :          3 :                         new |= RWSEM_FLAG_HANDOFF;
     584                 :            :                 } else {
     585                 :          3 :                         new |= RWSEM_WRITER_LOCKED;
     586                 :          3 :                         new &= ~RWSEM_FLAG_HANDOFF;
     587                 :            : 
     588                 :          3 :                         if (list_is_singular(&sem->wait_list))
     589                 :          3 :                                 new &= ~RWSEM_FLAG_WAITERS;
     590                 :            :                 }
     591                 :          3 :         } while (!atomic_long_try_cmpxchg_acquire(&sem->count, &count, new));
     592                 :            : 
     593                 :            :         /*
     594                 :            :          * We have either acquired the lock with handoff bit cleared or
     595                 :            :          * set the handoff bit.
     596                 :            :          */
     597                 :          3 :         if (new & RWSEM_FLAG_HANDOFF)
     598                 :            :                 return false;
     599                 :            : 
     600                 :            :         rwsem_set_owner(sem);
     601                 :          3 :         return true;
     602                 :            : }
     603                 :            : 
     604                 :            : #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
     605                 :            : /*
     606                 :            :  * Try to acquire read lock before the reader is put on wait queue.
     607                 :            :  * Lock acquisition isn't allowed if the rwsem is locked or a writer handoff
     608                 :            :  * is ongoing.
     609                 :            :  */
     610                 :          3 : static inline bool rwsem_try_read_lock_unqueued(struct rw_semaphore *sem)
     611                 :            : {
     612                 :            :         long count = atomic_long_read(&sem->count);
     613                 :            : 
     614                 :          3 :         if (count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))
     615                 :            :                 return false;
     616                 :            : 
     617                 :          3 :         count = atomic_long_fetch_add_acquire(RWSEM_READER_BIAS, &sem->count);
     618                 :          3 :         if (!(count & (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
     619                 :            :                 rwsem_set_reader_owned(sem);
     620                 :            :                 lockevent_inc(rwsem_opt_rlock);
     621                 :          3 :                 return true;
     622                 :            :         }
     623                 :            : 
     624                 :            :         /* Back out the change */
     625                 :            :         atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
     626                 :          3 :         return false;
     627                 :            : }
     628                 :            : 
     629                 :            : /*
     630                 :            :  * Try to acquire write lock before the writer has been put on wait queue.
     631                 :            :  */
     632                 :          3 : static inline bool rwsem_try_write_lock_unqueued(struct rw_semaphore *sem)
     633                 :            : {
     634                 :          3 :         long count = atomic_long_read(&sem->count);
     635                 :            : 
     636                 :          3 :         while (!(count & (RWSEM_LOCK_MASK|RWSEM_FLAG_HANDOFF))) {
     637                 :          3 :                 if (atomic_long_try_cmpxchg_acquire(&sem->count, &count,
     638                 :          3 :                                         count | RWSEM_WRITER_LOCKED)) {
     639                 :            :                         rwsem_set_owner(sem);
     640                 :            :                         lockevent_inc(rwsem_opt_wlock);
     641                 :          3 :                         return true;
     642                 :            :                 }
     643                 :            :         }
     644                 :            :         return false;
     645                 :            : }
     646                 :            : 
     647                 :            : static inline bool owner_on_cpu(struct task_struct *owner)
     648                 :            : {
     649                 :            :         /*
     650                 :            :          * As lock holder preemption issue, we both skip spinning if
     651                 :            :          * task is not on cpu or its cpu is preempted
     652                 :            :          */
     653                 :          3 :         return owner->on_cpu && !vcpu_is_preempted(task_cpu(owner));
     654                 :            : }
     655                 :            : 
     656                 :          3 : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
     657                 :            :                                            unsigned long nonspinnable)
     658                 :            : {
     659                 :            :         struct task_struct *owner;
     660                 :            :         unsigned long flags;
     661                 :            :         bool ret = true;
     662                 :            : 
     663                 :            :         BUILD_BUG_ON(!(RWSEM_OWNER_UNKNOWN & RWSEM_NONSPINNABLE));
     664                 :            : 
     665                 :          3 :         if (need_resched()) {
     666                 :            :                 lockevent_inc(rwsem_opt_fail);
     667                 :            :                 return false;
     668                 :            :         }
     669                 :            : 
     670                 :          3 :         preempt_disable();
     671                 :            :         rcu_read_lock();
     672                 :            :         owner = rwsem_owner_flags(sem, &flags);
     673                 :            :         /*
     674                 :            :          * Don't check the read-owner as the entry may be stale.
     675                 :            :          */
     676                 :          3 :         if ((flags & nonspinnable) ||
     677                 :          3 :             (owner && !(flags & RWSEM_READER_OWNED) && !owner_on_cpu(owner)))
     678                 :            :                 ret = false;
     679                 :            :         rcu_read_unlock();
     680                 :          3 :         preempt_enable();
     681                 :            : 
     682                 :            :         lockevent_cond_inc(rwsem_opt_fail, !ret);
     683                 :          3 :         return ret;
     684                 :            : }
     685                 :            : 
     686                 :            : /*
     687                 :            :  * The rwsem_spin_on_owner() function returns the folowing 4 values
     688                 :            :  * depending on the lock owner state.
     689                 :            :  *   OWNER_NULL  : owner is currently NULL
     690                 :            :  *   OWNER_WRITER: when owner changes and is a writer
     691                 :            :  *   OWNER_READER: when owner changes and the new owner may be a reader.
     692                 :            :  *   OWNER_NONSPINNABLE:
     693                 :            :  *                 when optimistic spinning has to stop because either the
     694                 :            :  *                 owner stops running, is unknown, or its timeslice has
     695                 :            :  *                 been used up.
     696                 :            :  */
     697                 :            : enum owner_state {
     698                 :            :         OWNER_NULL              = 1 << 0,
     699                 :            :         OWNER_WRITER            = 1 << 1,
     700                 :            :         OWNER_READER            = 1 << 2,
     701                 :            :         OWNER_NONSPINNABLE      = 1 << 3,
     702                 :            : };
     703                 :            : #define OWNER_SPINNABLE         (OWNER_NULL | OWNER_WRITER | OWNER_READER)
     704                 :            : 
     705                 :            : static inline enum owner_state
     706                 :            : rwsem_owner_state(struct task_struct *owner, unsigned long flags, unsigned long nonspinnable)
     707                 :            : {
     708                 :          3 :         if (flags & nonspinnable)
     709                 :            :                 return OWNER_NONSPINNABLE;
     710                 :            : 
     711                 :          3 :         if (flags & RWSEM_READER_OWNED)
     712                 :            :                 return OWNER_READER;
     713                 :            : 
     714                 :          3 :         return owner ? OWNER_WRITER : OWNER_NULL;
     715                 :            : }
     716                 :            : 
     717                 :            : static noinline enum owner_state
     718                 :          3 : rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
     719                 :            : {
     720                 :            :         struct task_struct *new, *owner;
     721                 :            :         unsigned long flags, new_flags;
     722                 :            :         enum owner_state state;
     723                 :            : 
     724                 :            :         owner = rwsem_owner_flags(sem, &flags);
     725                 :            :         state = rwsem_owner_state(owner, flags, nonspinnable);
     726                 :          3 :         if (state != OWNER_WRITER)
     727                 :            :                 return state;
     728                 :            : 
     729                 :            :         rcu_read_lock();
     730                 :            :         for (;;) {
     731                 :            :                 /*
     732                 :            :                  * When a waiting writer set the handoff flag, it may spin
     733                 :            :                  * on the owner as well. Once that writer acquires the lock,
     734                 :            :                  * we can spin on it. So we don't need to quit even when the
     735                 :            :                  * handoff bit is set.
     736                 :            :                  */
     737                 :            :                 new = rwsem_owner_flags(sem, &new_flags);
     738                 :          3 :                 if ((new != owner) || (new_flags != flags)) {
     739                 :          3 :                         state = rwsem_owner_state(new, new_flags, nonspinnable);
     740                 :          3 :                         break;
     741                 :            :                 }
     742                 :            : 
     743                 :            :                 /*
     744                 :            :                  * Ensure we emit the owner->on_cpu, dereference _after_
     745                 :            :                  * checking sem->owner still matches owner, if that fails,
     746                 :            :                  * owner might point to free()d memory, if it still matches,
     747                 :            :                  * the rcu_read_lock() ensures the memory stays valid.
     748                 :            :                  */
     749                 :          3 :                 barrier();
     750                 :            : 
     751                 :          3 :                 if (need_resched() || !owner_on_cpu(owner)) {
     752                 :            :                         state = OWNER_NONSPINNABLE;
     753                 :            :                         break;
     754                 :            :                 }
     755                 :            : 
     756                 :          3 :                 cpu_relax();
     757                 :          3 :         }
     758                 :            :         rcu_read_unlock();
     759                 :            : 
     760                 :          3 :         return state;
     761                 :            : }
     762                 :            : 
     763                 :            : /*
     764                 :            :  * Calculate reader-owned rwsem spinning threshold for writer
     765                 :            :  *
     766                 :            :  * The more readers own the rwsem, the longer it will take for them to
     767                 :            :  * wind down and free the rwsem. So the empirical formula used to
     768                 :            :  * determine the actual spinning time limit here is:
     769                 :            :  *
     770                 :            :  *   Spinning threshold = (10 + nr_readers/2)us
     771                 :            :  *
     772                 :            :  * The limit is capped to a maximum of 25us (30 readers). This is just
     773                 :            :  * a heuristic and is subjected to change in the future.
     774                 :            :  */
     775                 :          3 : static inline u64 rwsem_rspin_threshold(struct rw_semaphore *sem)
     776                 :            : {
     777                 :            :         long count = atomic_long_read(&sem->count);
     778                 :          3 :         int readers = count >> RWSEM_READER_SHIFT;
     779                 :            :         u64 delta;
     780                 :            : 
     781                 :          3 :         if (readers > 30)
     782                 :            :                 readers = 30;
     783                 :          3 :         delta = (20 + readers) * NSEC_PER_USEC / 2;
     784                 :            : 
     785                 :          3 :         return sched_clock() + delta;
     786                 :            : }
     787                 :            : 
     788                 :          3 : static bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
     789                 :            : {
     790                 :            :         bool taken = false;
     791                 :            :         int prev_owner_state = OWNER_NULL;
     792                 :            :         int loop = 0;
     793                 :            :         u64 rspin_threshold = 0;
     794                 :            :         unsigned long nonspinnable = wlock ? RWSEM_WR_NONSPINNABLE
     795                 :          3 :                                            : RWSEM_RD_NONSPINNABLE;
     796                 :            : 
     797                 :          3 :         preempt_disable();
     798                 :            : 
     799                 :            :         /* sem->wait_lock should not be held when doing optimistic spinning */
     800                 :          3 :         if (!osq_lock(&sem->osq))
     801                 :            :                 goto done;
     802                 :            : 
     803                 :            :         /*
     804                 :            :          * Optimistically spin on the owner field and attempt to acquire the
     805                 :            :          * lock whenever the owner changes. Spinning will be stopped when:
     806                 :            :          *  1) the owning writer isn't running; or
     807                 :            :          *  2) readers own the lock and spinning time has exceeded limit.
     808                 :            :          */
     809                 :            :         for (;;) {
     810                 :            :                 enum owner_state owner_state;
     811                 :            : 
     812                 :          3 :                 owner_state = rwsem_spin_on_owner(sem, nonspinnable);
     813                 :          3 :                 if (!(owner_state & OWNER_SPINNABLE))
     814                 :            :                         break;
     815                 :            : 
     816                 :            :                 /*
     817                 :            :                  * Try to acquire the lock
     818                 :            :                  */
     819                 :          3 :                 taken = wlock ? rwsem_try_write_lock_unqueued(sem)
     820                 :          3 :                               : rwsem_try_read_lock_unqueued(sem);
     821                 :            : 
     822                 :          3 :                 if (taken)
     823                 :            :                         break;
     824                 :            : 
     825                 :            :                 /*
     826                 :            :                  * Time-based reader-owned rwsem optimistic spinning
     827                 :            :                  */
     828                 :          3 :                 if (wlock && (owner_state == OWNER_READER)) {
     829                 :            :                         /*
     830                 :            :                          * Re-initialize rspin_threshold every time when
     831                 :            :                          * the owner state changes from non-reader to reader.
     832                 :            :                          * This allows a writer to steal the lock in between
     833                 :            :                          * 2 reader phases and have the threshold reset at
     834                 :            :                          * the beginning of the 2nd reader phase.
     835                 :            :                          */
     836                 :          3 :                         if (prev_owner_state != OWNER_READER) {
     837                 :          3 :                                 if (rwsem_test_oflags(sem, nonspinnable))
     838                 :            :                                         break;
     839                 :          3 :                                 rspin_threshold = rwsem_rspin_threshold(sem);
     840                 :            :                                 loop = 0;
     841                 :            :                         }
     842                 :            : 
     843                 :            :                         /*
     844                 :            :                          * Check time threshold once every 16 iterations to
     845                 :            :                          * avoid calling sched_clock() too frequently so
     846                 :            :                          * as to reduce the average latency between the times
     847                 :            :                          * when the lock becomes free and when the spinner
     848                 :            :                          * is ready to do a trylock.
     849                 :            :                          */
     850                 :          3 :                         else if (!(++loop & 0xf) && (sched_clock() > rspin_threshold)) {
     851                 :          3 :                                 rwsem_set_nonspinnable(sem);
     852                 :            :                                 lockevent_inc(rwsem_opt_nospin);
     853                 :          3 :                                 break;
     854                 :            :                         }
     855                 :            :                 }
     856                 :            : 
     857                 :            :                 /*
     858                 :            :                  * An RT task cannot do optimistic spinning if it cannot
     859                 :            :                  * be sure the lock holder is running or live-lock may
     860                 :            :                  * happen if the current task and the lock holder happen
     861                 :            :                  * to run in the same CPU. However, aborting optimistic
     862                 :            :                  * spinning while a NULL owner is detected may miss some
     863                 :            :                  * opportunity where spinning can continue without causing
     864                 :            :                  * problem.
     865                 :            :                  *
     866                 :            :                  * There are 2 possible cases where an RT task may be able
     867                 :            :                  * to continue spinning.
     868                 :            :                  *
     869                 :            :                  * 1) The lock owner is in the process of releasing the
     870                 :            :                  *    lock, sem->owner is cleared but the lock has not
     871                 :            :                  *    been released yet.
     872                 :            :                  * 2) The lock was free and owner cleared, but another
     873                 :            :                  *    task just comes in and acquire the lock before
     874                 :            :                  *    we try to get it. The new owner may be a spinnable
     875                 :            :                  *    writer.
     876                 :            :                  *
     877                 :            :                  * To take advantage of two scenarios listed agove, the RT
     878                 :            :                  * task is made to retry one more time to see if it can
     879                 :            :                  * acquire the lock or continue spinning on the new owning
     880                 :            :                  * writer. Of course, if the time lag is long enough or the
     881                 :            :                  * new owner is not a writer or spinnable, the RT task will
     882                 :            :                  * quit spinning.
     883                 :            :                  *
     884                 :            :                  * If the owner is a writer, the need_resched() check is
     885                 :            :                  * done inside rwsem_spin_on_owner(). If the owner is not
     886                 :            :                  * a writer, need_resched() check needs to be done here.
     887                 :            :                  */
     888                 :          3 :                 if (owner_state != OWNER_WRITER) {
     889                 :          3 :                         if (need_resched())
     890                 :            :                                 break;
     891                 :          3 :                         if (rt_task(current) &&
     892                 :            :                            (prev_owner_state != OWNER_WRITER))
     893                 :            :                                 break;
     894                 :            :                 }
     895                 :          3 :                 prev_owner_state = owner_state;
     896                 :            : 
     897                 :            :                 /*
     898                 :            :                  * The cpu_relax() call is a compiler barrier which forces
     899                 :            :                  * everything in this loop to be re-loaded. We don't need
     900                 :            :                  * memory barriers as we'll eventually observe the right
     901                 :            :                  * values at the cost of a few extra spins.
     902                 :            :                  */
     903                 :          3 :                 cpu_relax();
     904                 :          3 :         }
     905                 :          3 :         osq_unlock(&sem->osq);
     906                 :            : done:
     907                 :          3 :         preempt_enable();
     908                 :            :         lockevent_cond_inc(rwsem_opt_fail, !taken);
     909                 :          3 :         return taken;
     910                 :            : }
     911                 :            : 
     912                 :            : /*
     913                 :            :  * Clear the owner's RWSEM_WR_NONSPINNABLE bit if it is set. This should
     914                 :            :  * only be called when the reader count reaches 0.
     915                 :            :  *
     916                 :            :  * This give writers better chance to acquire the rwsem first before
     917                 :            :  * readers when the rwsem was being held by readers for a relatively long
     918                 :            :  * period of time. Race can happen that an optimistic spinner may have
     919                 :            :  * just stolen the rwsem and set the owner, but just clearing the
     920                 :            :  * RWSEM_WR_NONSPINNABLE bit will do no harm anyway.
     921                 :            :  */
     922                 :          3 : static inline void clear_wr_nonspinnable(struct rw_semaphore *sem)
     923                 :            : {
     924                 :          3 :         if (rwsem_test_oflags(sem, RWSEM_WR_NONSPINNABLE))
     925                 :          3 :                 atomic_long_andnot(RWSEM_WR_NONSPINNABLE, &sem->owner);
     926                 :          3 : }
     927                 :            : 
     928                 :            : /*
     929                 :            :  * This function is called when the reader fails to acquire the lock via
     930                 :            :  * optimistic spinning. In this case we will still attempt to do a trylock
     931                 :            :  * when comparing the rwsem state right now with the state when entering
     932                 :            :  * the slowpath indicates that the reader is still in a valid reader phase.
     933                 :            :  * This happens when the following conditions are true:
     934                 :            :  *
     935                 :            :  * 1) The lock is currently reader owned, and
     936                 :            :  * 2) The lock is previously not reader-owned or the last read owner changes.
     937                 :            :  *
     938                 :            :  * In the former case, we have transitioned from a writer phase to a
     939                 :            :  * reader-phase while spinning. In the latter case, it means the reader
     940                 :            :  * phase hasn't ended when we entered the optimistic spinning loop. In
     941                 :            :  * both cases, the reader is eligible to acquire the lock. This is the
     942                 :            :  * secondary path where a read lock is acquired optimistically.
     943                 :            :  *
     944                 :            :  * The reader non-spinnable bit wasn't set at time of entry or it will
     945                 :            :  * not be here at all.
     946                 :            :  */
     947                 :          3 : static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
     948                 :            :                                               unsigned long last_rowner)
     949                 :            : {
     950                 :          3 :         unsigned long owner = atomic_long_read(&sem->owner);
     951                 :            : 
     952                 :          3 :         if (!(owner & RWSEM_READER_OWNED))
     953                 :            :                 return false;
     954                 :            : 
     955                 :          3 :         if (((owner ^ last_rowner) & ~RWSEM_OWNER_FLAGS_MASK) &&
     956                 :          3 :             rwsem_try_read_lock_unqueued(sem)) {
     957                 :            :                 lockevent_inc(rwsem_opt_rlock2);
     958                 :            :                 lockevent_add(rwsem_opt_fail, -1);
     959                 :            :                 return true;
     960                 :            :         }
     961                 :            :         return false;
     962                 :            : }
     963                 :            : #else
     964                 :            : static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem,
     965                 :            :                                            unsigned long nonspinnable)
     966                 :            : {
     967                 :            :         return false;
     968                 :            : }
     969                 :            : 
     970                 :            : static inline bool rwsem_optimistic_spin(struct rw_semaphore *sem, bool wlock)
     971                 :            : {
     972                 :            :         return false;
     973                 :            : }
     974                 :            : 
     975                 :            : static inline void clear_wr_nonspinnable(struct rw_semaphore *sem) { }
     976                 :            : 
     977                 :            : static inline bool rwsem_reader_phase_trylock(struct rw_semaphore *sem,
     978                 :            :                                               unsigned long last_rowner)
     979                 :            : {
     980                 :            :         return false;
     981                 :            : }
     982                 :            : 
     983                 :            : static inline int
     984                 :            : rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
     985                 :            : {
     986                 :            :         return 0;
     987                 :            : }
     988                 :            : #define OWNER_NULL      1
     989                 :            : #endif
     990                 :            : 
     991                 :            : /*
     992                 :            :  * Wait for the read lock to be granted
     993                 :            :  */
     994                 :            : static struct rw_semaphore __sched *
     995                 :          3 : rwsem_down_read_slowpath(struct rw_semaphore *sem, int state)
     996                 :            : {
     997                 :            :         long count, adjustment = -RWSEM_READER_BIAS;
     998                 :            :         struct rwsem_waiter waiter;
     999                 :          3 :         DEFINE_WAKE_Q(wake_q);
    1000                 :            :         bool wake = false;
    1001                 :            : 
    1002                 :            :         /*
    1003                 :            :          * Save the current read-owner of rwsem, if available, and the
    1004                 :            :          * reader nonspinnable bit.
    1005                 :            :          */
    1006                 :          3 :         waiter.last_rowner = atomic_long_read(&sem->owner);
    1007                 :          3 :         if (!(waiter.last_rowner & RWSEM_READER_OWNED))
    1008                 :          3 :                 waiter.last_rowner &= RWSEM_RD_NONSPINNABLE;
    1009                 :            : 
    1010                 :          3 :         if (!rwsem_can_spin_on_owner(sem, RWSEM_RD_NONSPINNABLE))
    1011                 :            :                 goto queue;
    1012                 :            : 
    1013                 :            :         /*
    1014                 :            :          * Undo read bias from down_read() and do optimistic spinning.
    1015                 :            :          */
    1016                 :          3 :         atomic_long_add(-RWSEM_READER_BIAS, &sem->count);
    1017                 :            :         adjustment = 0;
    1018                 :          3 :         if (rwsem_optimistic_spin(sem, false)) {
    1019                 :            :                 /* rwsem_optimistic_spin() implies ACQUIRE on success */
    1020                 :            :                 /*
    1021                 :            :                  * Wake up other readers in the wait list if the front
    1022                 :            :                  * waiter is a reader.
    1023                 :            :                  */
    1024                 :          3 :                 if ((atomic_long_read(&sem->count) & RWSEM_FLAG_WAITERS)) {
    1025                 :          3 :                         raw_spin_lock_irq(&sem->wait_lock);
    1026                 :          3 :                         if (!list_empty(&sem->wait_list))
    1027                 :          3 :                                 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED,
    1028                 :            :                                                 &wake_q);
    1029                 :          3 :                         raw_spin_unlock_irq(&sem->wait_lock);
    1030                 :          3 :                         wake_up_q(&wake_q);
    1031                 :            :                 }
    1032                 :          3 :                 return sem;
    1033                 :          3 :         } else if (rwsem_reader_phase_trylock(sem, waiter.last_rowner)) {
    1034                 :            :                 /* rwsem_reader_phase_trylock() implies ACQUIRE on success */
    1035                 :            :                 return sem;
    1036                 :            :         }
    1037                 :            : 
    1038                 :            : queue:
    1039                 :          3 :         waiter.task = current;
    1040                 :          3 :         waiter.type = RWSEM_WAITING_FOR_READ;
    1041                 :          3 :         waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
    1042                 :            : 
    1043                 :          3 :         raw_spin_lock_irq(&sem->wait_lock);
    1044                 :          3 :         if (list_empty(&sem->wait_list)) {
    1045                 :            :                 /*
    1046                 :            :                  * In case the wait queue is empty and the lock isn't owned
    1047                 :            :                  * by a writer or has the handoff bit set, this reader can
    1048                 :            :                  * exit the slowpath and return immediately as its
    1049                 :            :                  * RWSEM_READER_BIAS has already been set in the count.
    1050                 :            :                  */
    1051                 :          3 :                 if (adjustment && !(atomic_long_read(&sem->count) &
    1052                 :            :                      (RWSEM_WRITER_MASK | RWSEM_FLAG_HANDOFF))) {
    1053                 :            :                         /* Provide lock ACQUIRE */
    1054                 :          3 :                         smp_acquire__after_ctrl_dep();
    1055                 :          3 :                         raw_spin_unlock_irq(&sem->wait_lock);
    1056                 :            :                         rwsem_set_reader_owned(sem);
    1057                 :            :                         lockevent_inc(rwsem_rlock_fast);
    1058                 :          3 :                         return sem;
    1059                 :            :                 }
    1060                 :          3 :                 adjustment += RWSEM_FLAG_WAITERS;
    1061                 :            :         }
    1062                 :            :         list_add_tail(&waiter.list, &sem->wait_list);
    1063                 :            : 
    1064                 :            :         /* we're now waiting on the lock, but no longer actively locking */
    1065                 :          3 :         if (adjustment)
    1066                 :          3 :                 count = atomic_long_add_return(adjustment, &sem->count);
    1067                 :            :         else
    1068                 :            :                 count = atomic_long_read(&sem->count);
    1069                 :            : 
    1070                 :            :         /*
    1071                 :            :          * If there are no active locks, wake the front queued process(es).
    1072                 :            :          *
    1073                 :            :          * If there are no writers and we are first in the queue,
    1074                 :            :          * wake our own waiter to join the existing active readers !
    1075                 :            :          */
    1076                 :          3 :         if (!(count & RWSEM_LOCK_MASK)) {
    1077                 :          3 :                 clear_wr_nonspinnable(sem);
    1078                 :            :                 wake = true;
    1079                 :            :         }
    1080                 :          3 :         if (wake || (!(count & RWSEM_WRITER_MASK) &&
    1081                 :          3 :                     (adjustment & RWSEM_FLAG_WAITERS)))
    1082                 :          3 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
    1083                 :            : 
    1084                 :          3 :         raw_spin_unlock_irq(&sem->wait_lock);
    1085                 :          3 :         wake_up_q(&wake_q);
    1086                 :            : 
    1087                 :            :         /* wait to be given the lock */
    1088                 :            :         for (;;) {
    1089                 :          3 :                 set_current_state(state);
    1090                 :          3 :                 if (!smp_load_acquire(&waiter.task)) {
    1091                 :            :                         /* Matches rwsem_mark_wake()'s smp_store_release(). */
    1092                 :            :                         break;
    1093                 :            :                 }
    1094                 :          3 :                 if (signal_pending_state(state, current)) {
    1095                 :          0 :                         raw_spin_lock_irq(&sem->wait_lock);
    1096                 :          0 :                         if (waiter.task)
    1097                 :            :                                 goto out_nolock;
    1098                 :          0 :                         raw_spin_unlock_irq(&sem->wait_lock);
    1099                 :            :                         /* Ordered by sem->wait_lock against rwsem_mark_wake(). */
    1100                 :          0 :                         break;
    1101                 :            :                 }
    1102                 :          3 :                 schedule();
    1103                 :            :                 lockevent_inc(rwsem_sleep_reader);
    1104                 :          3 :         }
    1105                 :            : 
    1106                 :          3 :         __set_current_state(TASK_RUNNING);
    1107                 :            :         lockevent_inc(rwsem_rlock);
    1108                 :          3 :         return sem;
    1109                 :            : 
    1110                 :            : out_nolock:
    1111                 :            :         list_del(&waiter.list);
    1112                 :          0 :         if (list_empty(&sem->wait_list)) {
    1113                 :          0 :                 atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF,
    1114                 :            :                                    &sem->count);
    1115                 :            :         }
    1116                 :          0 :         raw_spin_unlock_irq(&sem->wait_lock);
    1117                 :          0 :         __set_current_state(TASK_RUNNING);
    1118                 :            :         lockevent_inc(rwsem_rlock_fail);
    1119                 :          0 :         return ERR_PTR(-EINTR);
    1120                 :            : }
    1121                 :            : 
    1122                 :            : /*
    1123                 :            :  * This function is called by the a write lock owner. So the owner value
    1124                 :            :  * won't get changed by others.
    1125                 :            :  */
    1126                 :          3 : static inline void rwsem_disable_reader_optspin(struct rw_semaphore *sem,
    1127                 :            :                                                 bool disable)
    1128                 :            : {
    1129                 :          3 :         if (unlikely(disable)) {
    1130                 :          3 :                 atomic_long_or(RWSEM_RD_NONSPINNABLE, &sem->owner);
    1131                 :            :                 lockevent_inc(rwsem_opt_norspin);
    1132                 :            :         }
    1133                 :          3 : }
    1134                 :            : 
    1135                 :            : /*
    1136                 :            :  * Wait until we successfully acquire the write lock
    1137                 :            :  */
    1138                 :            : static struct rw_semaphore *
    1139                 :          3 : rwsem_down_write_slowpath(struct rw_semaphore *sem, int state)
    1140                 :            : {
    1141                 :            :         long count;
    1142                 :            :         bool disable_rspin;
    1143                 :            :         enum writer_wait_state wstate;
    1144                 :            :         struct rwsem_waiter waiter;
    1145                 :            :         struct rw_semaphore *ret = sem;
    1146                 :          3 :         DEFINE_WAKE_Q(wake_q);
    1147                 :            : 
    1148                 :            :         /* do optimistic spinning and steal lock if possible */
    1149                 :          3 :         if (rwsem_can_spin_on_owner(sem, RWSEM_WR_NONSPINNABLE) &&
    1150                 :          3 :             rwsem_optimistic_spin(sem, true)) {
    1151                 :            :                 /* rwsem_optimistic_spin() implies ACQUIRE on success */
    1152                 :            :                 return sem;
    1153                 :            :         }
    1154                 :            : 
    1155                 :            :         /*
    1156                 :            :          * Disable reader optimistic spinning for this rwsem after
    1157                 :            :          * acquiring the write lock when the setting of the nonspinnable
    1158                 :            :          * bits are observed.
    1159                 :            :          */
    1160                 :          3 :         disable_rspin = atomic_long_read(&sem->owner) & RWSEM_NONSPINNABLE;
    1161                 :            : 
    1162                 :            :         /*
    1163                 :            :          * Optimistic spinning failed, proceed to the slowpath
    1164                 :            :          * and block until we can acquire the sem.
    1165                 :            :          */
    1166                 :          3 :         waiter.task = current;
    1167                 :          3 :         waiter.type = RWSEM_WAITING_FOR_WRITE;
    1168                 :          3 :         waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT;
    1169                 :            : 
    1170                 :          3 :         raw_spin_lock_irq(&sem->wait_lock);
    1171                 :            : 
    1172                 :            :         /* account for this before adding a new element to the list */
    1173                 :          3 :         wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST;
    1174                 :            : 
    1175                 :            :         list_add_tail(&waiter.list, &sem->wait_list);
    1176                 :            : 
    1177                 :            :         /* we're now waiting on the lock */
    1178                 :          3 :         if (wstate == WRITER_NOT_FIRST) {
    1179                 :            :                 count = atomic_long_read(&sem->count);
    1180                 :            : 
    1181                 :            :                 /*
    1182                 :            :                  * If there were already threads queued before us and:
    1183                 :            :                  *  1) there are no no active locks, wake the front
    1184                 :            :                  *     queued process(es) as the handoff bit might be set.
    1185                 :            :                  *  2) there are no active writers and some readers, the lock
    1186                 :            :                  *     must be read owned; so we try to wake any read lock
    1187                 :            :                  *     waiters that were queued ahead of us.
    1188                 :            :                  */
    1189                 :          3 :                 if (count & RWSEM_WRITER_MASK)
    1190                 :            :                         goto wait;
    1191                 :            : 
    1192                 :          3 :                 rwsem_mark_wake(sem, (count & RWSEM_READER_MASK)
    1193                 :            :                                         ? RWSEM_WAKE_READERS
    1194                 :            :                                         : RWSEM_WAKE_ANY, &wake_q);
    1195                 :            : 
    1196                 :          3 :                 if (!wake_q_empty(&wake_q)) {
    1197                 :            :                         /*
    1198                 :            :                          * We want to minimize wait_lock hold time especially
    1199                 :            :                          * when a large number of readers are to be woken up.
    1200                 :            :                          */
    1201                 :          3 :                         raw_spin_unlock_irq(&sem->wait_lock);
    1202                 :          3 :                         wake_up_q(&wake_q);
    1203                 :            :                         wake_q_init(&wake_q);       /* Used again, reinit */
    1204                 :          3 :                         raw_spin_lock_irq(&sem->wait_lock);
    1205                 :            :                 }
    1206                 :            :         } else {
    1207                 :          3 :                 atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count);
    1208                 :            :         }
    1209                 :            : 
    1210                 :            : wait:
    1211                 :            :         /* wait until we successfully acquire the lock */
    1212                 :          3 :         set_current_state(state);
    1213                 :            :         for (;;) {
    1214                 :          3 :                 if (rwsem_try_write_lock(sem, wstate)) {
    1215                 :            :                         /* rwsem_try_write_lock() implies ACQUIRE on success */
    1216                 :            :                         break;
    1217                 :            :                 }
    1218                 :            : 
    1219                 :          3 :                 raw_spin_unlock_irq(&sem->wait_lock);
    1220                 :            : 
    1221                 :            :                 /*
    1222                 :            :                  * After setting the handoff bit and failing to acquire
    1223                 :            :                  * the lock, attempt to spin on owner to accelerate lock
    1224                 :            :                  * transfer. If the previous owner is a on-cpu writer and it
    1225                 :            :                  * has just released the lock, OWNER_NULL will be returned.
    1226                 :            :                  * In this case, we attempt to acquire the lock again
    1227                 :            :                  * without sleeping.
    1228                 :            :                  */
    1229                 :          3 :                 if (wstate == WRITER_HANDOFF &&
    1230                 :          3 :                     rwsem_spin_on_owner(sem, RWSEM_NONSPINNABLE) == OWNER_NULL)
    1231                 :            :                         goto trylock_again;
    1232                 :            : 
    1233                 :            :                 /* Block until there are no active lockers. */
    1234                 :            :                 for (;;) {
    1235                 :          3 :                         if (signal_pending_state(state, current))
    1236                 :            :                                 goto out_nolock;
    1237                 :            : 
    1238                 :          3 :                         schedule();
    1239                 :            :                         lockevent_inc(rwsem_sleep_writer);
    1240                 :          3 :                         set_current_state(state);
    1241                 :            :                         /*
    1242                 :            :                          * If HANDOFF bit is set, unconditionally do
    1243                 :            :                          * a trylock.
    1244                 :            :                          */
    1245                 :          3 :                         if (wstate == WRITER_HANDOFF)
    1246                 :            :                                 break;
    1247                 :            : 
    1248                 :          3 :                         if ((wstate == WRITER_NOT_FIRST) &&
    1249                 :          3 :                             (rwsem_first_waiter(sem) == &waiter))
    1250                 :            :                                 wstate = WRITER_FIRST;
    1251                 :            : 
    1252                 :            :                         count = atomic_long_read(&sem->count);
    1253                 :          3 :                         if (!(count & RWSEM_LOCK_MASK))
    1254                 :            :                                 break;
    1255                 :            : 
    1256                 :            :                         /*
    1257                 :            :                          * The setting of the handoff bit is deferred
    1258                 :            :                          * until rwsem_try_write_lock() is called.
    1259                 :            :                          */
    1260                 :          3 :                         if ((wstate == WRITER_FIRST) && (rt_task(current) ||
    1261                 :          3 :                             time_after(jiffies, waiter.timeout))) {
    1262                 :            :                                 wstate = WRITER_HANDOFF;
    1263                 :            :                                 lockevent_inc(rwsem_wlock_handoff);
    1264                 :            :                                 break;
    1265                 :            :                         }
    1266                 :            :                 }
    1267                 :            : trylock_again:
    1268                 :          3 :                 raw_spin_lock_irq(&sem->wait_lock);
    1269                 :          3 :         }
    1270                 :          3 :         __set_current_state(TASK_RUNNING);
    1271                 :            :         list_del(&waiter.list);
    1272                 :          3 :         rwsem_disable_reader_optspin(sem, disable_rspin);
    1273                 :          3 :         raw_spin_unlock_irq(&sem->wait_lock);
    1274                 :            :         lockevent_inc(rwsem_wlock);
    1275                 :            : 
    1276                 :          3 :         return ret;
    1277                 :            : 
    1278                 :            : out_nolock:
    1279                 :          0 :         __set_current_state(TASK_RUNNING);
    1280                 :          0 :         raw_spin_lock_irq(&sem->wait_lock);
    1281                 :            :         list_del(&waiter.list);
    1282                 :            : 
    1283                 :          0 :         if (unlikely(wstate == WRITER_HANDOFF))
    1284                 :          0 :                 atomic_long_add(-RWSEM_FLAG_HANDOFF,  &sem->count);
    1285                 :            : 
    1286                 :          0 :         if (list_empty(&sem->wait_list))
    1287                 :          0 :                 atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count);
    1288                 :            :         else
    1289                 :          0 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
    1290                 :          0 :         raw_spin_unlock_irq(&sem->wait_lock);
    1291                 :          0 :         wake_up_q(&wake_q);
    1292                 :            :         lockevent_inc(rwsem_wlock_fail);
    1293                 :            : 
    1294                 :          0 :         return ERR_PTR(-EINTR);
    1295                 :            : }
    1296                 :            : 
    1297                 :            : /*
    1298                 :            :  * handle waking up a waiter on the semaphore
    1299                 :            :  * - up_read/up_write has decremented the active part of count if we come here
    1300                 :            :  */
    1301                 :          3 : static struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem, long count)
    1302                 :            : {
    1303                 :            :         unsigned long flags;
    1304                 :          3 :         DEFINE_WAKE_Q(wake_q);
    1305                 :            : 
    1306                 :          3 :         raw_spin_lock_irqsave(&sem->wait_lock, flags);
    1307                 :            : 
    1308                 :          3 :         if (!list_empty(&sem->wait_list))
    1309                 :          3 :                 rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q);
    1310                 :            : 
    1311                 :          3 :         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    1312                 :          3 :         wake_up_q(&wake_q);
    1313                 :            : 
    1314                 :          3 :         return sem;
    1315                 :            : }
    1316                 :            : 
    1317                 :            : /*
    1318                 :            :  * downgrade a write lock into a read lock
    1319                 :            :  * - caller incremented waiting part of count and discovered it still negative
    1320                 :            :  * - just wake up any readers at the front of the queue
    1321                 :            :  */
    1322                 :          3 : static struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
    1323                 :            : {
    1324                 :            :         unsigned long flags;
    1325                 :          3 :         DEFINE_WAKE_Q(wake_q);
    1326                 :            : 
    1327                 :          3 :         raw_spin_lock_irqsave(&sem->wait_lock, flags);
    1328                 :            : 
    1329                 :          3 :         if (!list_empty(&sem->wait_list))
    1330                 :          3 :                 rwsem_mark_wake(sem, RWSEM_WAKE_READ_OWNED, &wake_q);
    1331                 :            : 
    1332                 :          3 :         raw_spin_unlock_irqrestore(&sem->wait_lock, flags);
    1333                 :          3 :         wake_up_q(&wake_q);
    1334                 :            : 
    1335                 :          3 :         return sem;
    1336                 :            : }
    1337                 :            : 
    1338                 :            : /*
    1339                 :            :  * lock for reading
    1340                 :            :  */
    1341                 :          3 : inline void __down_read(struct rw_semaphore *sem)
    1342                 :            : {
    1343                 :          3 :         if (!rwsem_read_trylock(sem)) {
    1344                 :          3 :                 rwsem_down_read_slowpath(sem, TASK_UNINTERRUPTIBLE);
    1345                 :            :                 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1346                 :            :         } else {
    1347                 :            :                 rwsem_set_reader_owned(sem);
    1348                 :            :         }
    1349                 :          3 : }
    1350                 :            : 
    1351                 :          3 : static inline int __down_read_killable(struct rw_semaphore *sem)
    1352                 :            : {
    1353                 :          3 :         if (!rwsem_read_trylock(sem)) {
    1354                 :          3 :                 if (IS_ERR(rwsem_down_read_slowpath(sem, TASK_KILLABLE)))
    1355                 :            :                         return -EINTR;
    1356                 :            :                 DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1357                 :            :         } else {
    1358                 :            :                 rwsem_set_reader_owned(sem);
    1359                 :            :         }
    1360                 :            :         return 0;
    1361                 :            : }
    1362                 :            : 
    1363                 :          3 : static inline int __down_read_trylock(struct rw_semaphore *sem)
    1364                 :            : {
    1365                 :            :         long tmp;
    1366                 :            : 
    1367                 :            :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1368                 :            : 
    1369                 :            :         /*
    1370                 :            :          * Optimize for the case when the rwsem is not locked at all.
    1371                 :            :          */
    1372                 :          3 :         tmp = RWSEM_UNLOCKED_VALUE;
    1373                 :            :         do {
    1374                 :          3 :                 if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
    1375                 :          3 :                                         tmp + RWSEM_READER_BIAS)) {
    1376                 :            :                         rwsem_set_reader_owned(sem);
    1377                 :          3 :                         return 1;
    1378                 :            :                 }
    1379                 :          3 :         } while (!(tmp & RWSEM_READ_FAILED_MASK));
    1380                 :            :         return 0;
    1381                 :            : }
    1382                 :            : 
    1383                 :            : /*
    1384                 :            :  * lock for writing
    1385                 :            :  */
    1386                 :          3 : static inline void __down_write(struct rw_semaphore *sem)
    1387                 :            : {
    1388                 :          3 :         long tmp = RWSEM_UNLOCKED_VALUE;
    1389                 :            : 
    1390                 :          3 :         if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
    1391                 :            :                                                       RWSEM_WRITER_LOCKED)))
    1392                 :          3 :                 rwsem_down_write_slowpath(sem, TASK_UNINTERRUPTIBLE);
    1393                 :            :         else
    1394                 :            :                 rwsem_set_owner(sem);
    1395                 :          3 : }
    1396                 :            : 
    1397                 :          3 : static inline int __down_write_killable(struct rw_semaphore *sem)
    1398                 :            : {
    1399                 :          3 :         long tmp = RWSEM_UNLOCKED_VALUE;
    1400                 :            : 
    1401                 :          3 :         if (unlikely(!atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
    1402                 :            :                                                       RWSEM_WRITER_LOCKED))) {
    1403                 :          3 :                 if (IS_ERR(rwsem_down_write_slowpath(sem, TASK_KILLABLE)))
    1404                 :            :                         return -EINTR;
    1405                 :            :         } else {
    1406                 :            :                 rwsem_set_owner(sem);
    1407                 :            :         }
    1408                 :            :         return 0;
    1409                 :            : }
    1410                 :            : 
    1411                 :          3 : static inline int __down_write_trylock(struct rw_semaphore *sem)
    1412                 :            : {
    1413                 :            :         long tmp;
    1414                 :            : 
    1415                 :            :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1416                 :            : 
    1417                 :          3 :         tmp  = RWSEM_UNLOCKED_VALUE;
    1418                 :          3 :         if (atomic_long_try_cmpxchg_acquire(&sem->count, &tmp,
    1419                 :            :                                             RWSEM_WRITER_LOCKED)) {
    1420                 :            :                 rwsem_set_owner(sem);
    1421                 :          3 :                 return true;
    1422                 :            :         }
    1423                 :            :         return false;
    1424                 :            : }
    1425                 :            : 
    1426                 :            : /*
    1427                 :            :  * unlock after reading
    1428                 :            :  */
    1429                 :          3 : inline void __up_read(struct rw_semaphore *sem)
    1430                 :            : {
    1431                 :            :         long tmp;
    1432                 :            : 
    1433                 :            :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1434                 :            :         DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1435                 :            : 
    1436                 :            :         rwsem_clear_reader_owned(sem);
    1437                 :          3 :         tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
    1438                 :            :         DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
    1439                 :          3 :         if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
    1440                 :            :                       RWSEM_FLAG_WAITERS)) {
    1441                 :          3 :                 clear_wr_nonspinnable(sem);
    1442                 :          3 :                 rwsem_wake(sem, tmp);
    1443                 :            :         }
    1444                 :          3 : }
    1445                 :            : 
    1446                 :            : /*
    1447                 :            :  * unlock after writing
    1448                 :            :  */
    1449                 :          3 : static inline void __up_write(struct rw_semaphore *sem)
    1450                 :            : {
    1451                 :            :         long tmp;
    1452                 :            : 
    1453                 :            :         DEBUG_RWSEMS_WARN_ON(sem->magic != sem, sem);
    1454                 :            :         /*
    1455                 :            :          * sem->owner may differ from current if the ownership is transferred
    1456                 :            :          * to an anonymous writer by setting the RWSEM_NONSPINNABLE bits.
    1457                 :            :          */
    1458                 :            :         DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) &&
    1459                 :            :                             !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE), sem);
    1460                 :            : 
    1461                 :            :         rwsem_clear_owner(sem);
    1462                 :          3 :         tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
    1463                 :          3 :         if (unlikely(tmp & RWSEM_FLAG_WAITERS))
    1464                 :          3 :                 rwsem_wake(sem, tmp);
    1465                 :          3 : }
    1466                 :            : 
    1467                 :            : /*
    1468                 :            :  * downgrade write lock to read lock
    1469                 :            :  */
    1470                 :          3 : static inline void __downgrade_write(struct rw_semaphore *sem)
    1471                 :            : {
    1472                 :            :         long tmp;
    1473                 :            : 
    1474                 :            :         /*
    1475                 :            :          * When downgrading from exclusive to shared ownership,
    1476                 :            :          * anything inside the write-locked region cannot leak
    1477                 :            :          * into the read side. In contrast, anything in the
    1478                 :            :          * read-locked region is ok to be re-ordered into the
    1479                 :            :          * write side. As such, rely on RELEASE semantics.
    1480                 :            :          */
    1481                 :            :         DEBUG_RWSEMS_WARN_ON(rwsem_owner(sem) != current, sem);
    1482                 :          3 :         tmp = atomic_long_fetch_add_release(
    1483                 :            :                 -RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
    1484                 :            :         rwsem_set_reader_owned(sem);
    1485                 :          3 :         if (tmp & RWSEM_FLAG_WAITERS)
    1486                 :          3 :                 rwsem_downgrade_wake(sem);
    1487                 :          3 : }
    1488                 :            : 
    1489                 :            : /*
    1490                 :            :  * lock for reading
    1491                 :            :  */
    1492                 :          3 : void __sched down_read(struct rw_semaphore *sem)
    1493                 :            : {
    1494                 :          3 :         might_sleep();
    1495                 :            :         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
    1496                 :            : 
    1497                 :          3 :         LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
    1498                 :          3 : }
    1499                 :            : EXPORT_SYMBOL(down_read);
    1500                 :            : 
    1501                 :          3 : int __sched down_read_killable(struct rw_semaphore *sem)
    1502                 :            : {
    1503                 :          3 :         might_sleep();
    1504                 :            :         rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
    1505                 :            : 
    1506                 :          3 :         if (LOCK_CONTENDED_RETURN(sem, __down_read_trylock, __down_read_killable)) {
    1507                 :            :                 rwsem_release(&sem->dep_map, 1, _RET_IP_);
    1508                 :            :                 return -EINTR;
    1509                 :            :         }
    1510                 :            : 
    1511                 :          3 :         return 0;
    1512                 :            : }
    1513                 :            : EXPORT_SYMBOL(down_read_killable);
    1514                 :            : 
    1515                 :            : /*
    1516                 :            :  * trylock for reading -- returns 1 if successful, 0 if contention
    1517                 :            :  */
    1518                 :          3 : int down_read_trylock(struct rw_semaphore *sem)
    1519                 :            : {
    1520                 :          3 :         int ret = __down_read_trylock(sem);
    1521                 :            : 
    1522                 :            :         if (ret == 1)
    1523                 :            :                 rwsem_acquire_read(&sem->dep_map, 0, 1, _RET_IP_);
    1524                 :          3 :         return ret;
    1525                 :            : }
    1526                 :            : EXPORT_SYMBOL(down_read_trylock);
    1527                 :            : 
    1528                 :            : /*
    1529                 :            :  * lock for writing
    1530                 :            :  */
    1531                 :          3 : void __sched down_write(struct rw_semaphore *sem)
    1532                 :            : {
    1533                 :          3 :         might_sleep();
    1534                 :            :         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
    1535                 :          3 :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1536                 :          3 : }
    1537                 :            : EXPORT_SYMBOL(down_write);
    1538                 :            : 
    1539                 :            : /*
    1540                 :            :  * lock for writing
    1541                 :            :  */
    1542                 :          3 : int __sched down_write_killable(struct rw_semaphore *sem)
    1543                 :            : {
    1544                 :          3 :         might_sleep();
    1545                 :            :         rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
    1546                 :            : 
    1547                 :          3 :         if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
    1548                 :            :                                   __down_write_killable)) {
    1549                 :            :                 rwsem_release(&sem->dep_map, 1, _RET_IP_);
    1550                 :            :                 return -EINTR;
    1551                 :            :         }
    1552                 :            : 
    1553                 :          3 :         return 0;
    1554                 :            : }
    1555                 :            : EXPORT_SYMBOL(down_write_killable);
    1556                 :            : 
    1557                 :            : /*
    1558                 :            :  * trylock for writing -- returns 1 if successful, 0 if contention
    1559                 :            :  */
    1560                 :          3 : int down_write_trylock(struct rw_semaphore *sem)
    1561                 :            : {
    1562                 :          3 :         int ret = __down_write_trylock(sem);
    1563                 :            : 
    1564                 :            :         if (ret == 1)
    1565                 :            :                 rwsem_acquire(&sem->dep_map, 0, 1, _RET_IP_);
    1566                 :            : 
    1567                 :          3 :         return ret;
    1568                 :            : }
    1569                 :            : EXPORT_SYMBOL(down_write_trylock);
    1570                 :            : 
    1571                 :            : /*
    1572                 :            :  * release a read lock
    1573                 :            :  */
    1574                 :          3 : void up_read(struct rw_semaphore *sem)
    1575                 :            : {
    1576                 :            :         rwsem_release(&sem->dep_map, 1, _RET_IP_);
    1577                 :          3 :         __up_read(sem);
    1578                 :          3 : }
    1579                 :            : EXPORT_SYMBOL(up_read);
    1580                 :            : 
    1581                 :            : /*
    1582                 :            :  * release a write lock
    1583                 :            :  */
    1584                 :          3 : void up_write(struct rw_semaphore *sem)
    1585                 :            : {
    1586                 :            :         rwsem_release(&sem->dep_map, 1, _RET_IP_);
    1587                 :          3 :         __up_write(sem);
    1588                 :          3 : }
    1589                 :            : EXPORT_SYMBOL(up_write);
    1590                 :            : 
    1591                 :            : /*
    1592                 :            :  * downgrade write lock to read lock
    1593                 :            :  */
    1594                 :          3 : void downgrade_write(struct rw_semaphore *sem)
    1595                 :            : {
    1596                 :            :         lock_downgrade(&sem->dep_map, _RET_IP_);
    1597                 :          3 :         __downgrade_write(sem);
    1598                 :          3 : }
    1599                 :            : EXPORT_SYMBOL(downgrade_write);
    1600                 :            : 
    1601                 :            : #ifdef CONFIG_DEBUG_LOCK_ALLOC
    1602                 :            : 
    1603                 :            : void down_read_nested(struct rw_semaphore *sem, int subclass)
    1604                 :            : {
    1605                 :            :         might_sleep();
    1606                 :            :         rwsem_acquire_read(&sem->dep_map, subclass, 0, _RET_IP_);
    1607                 :            :         LOCK_CONTENDED(sem, __down_read_trylock, __down_read);
    1608                 :            : }
    1609                 :            : EXPORT_SYMBOL(down_read_nested);
    1610                 :            : 
    1611                 :            : void _down_write_nest_lock(struct rw_semaphore *sem, struct lockdep_map *nest)
    1612                 :            : {
    1613                 :            :         might_sleep();
    1614                 :            :         rwsem_acquire_nest(&sem->dep_map, 0, 0, nest, _RET_IP_);
    1615                 :            :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1616                 :            : }
    1617                 :            : EXPORT_SYMBOL(_down_write_nest_lock);
    1618                 :            : 
    1619                 :            : void down_read_non_owner(struct rw_semaphore *sem)
    1620                 :            : {
    1621                 :            :         might_sleep();
    1622                 :            :         __down_read(sem);
    1623                 :            :         __rwsem_set_reader_owned(sem, NULL);
    1624                 :            : }
    1625                 :            : EXPORT_SYMBOL(down_read_non_owner);
    1626                 :            : 
    1627                 :            : void down_write_nested(struct rw_semaphore *sem, int subclass)
    1628                 :            : {
    1629                 :            :         might_sleep();
    1630                 :            :         rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
    1631                 :            :         LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
    1632                 :            : }
    1633                 :            : EXPORT_SYMBOL(down_write_nested);
    1634                 :            : 
    1635                 :            : int __sched down_write_killable_nested(struct rw_semaphore *sem, int subclass)
    1636                 :            : {
    1637                 :            :         might_sleep();
    1638                 :            :         rwsem_acquire(&sem->dep_map, subclass, 0, _RET_IP_);
    1639                 :            : 
    1640                 :            :         if (LOCK_CONTENDED_RETURN(sem, __down_write_trylock,
    1641                 :            :                                   __down_write_killable)) {
    1642                 :            :                 rwsem_release(&sem->dep_map, 1, _RET_IP_);
    1643                 :            :                 return -EINTR;
    1644                 :            :         }
    1645                 :            : 
    1646                 :            :         return 0;
    1647                 :            : }
    1648                 :            : EXPORT_SYMBOL(down_write_killable_nested);
    1649                 :            : 
    1650                 :            : void up_read_non_owner(struct rw_semaphore *sem)
    1651                 :            : {
    1652                 :            :         DEBUG_RWSEMS_WARN_ON(!is_rwsem_reader_owned(sem), sem);
    1653                 :            :         __up_read(sem);
    1654                 :            : }
    1655                 :            : EXPORT_SYMBOL(up_read_non_owner);
    1656                 :            : 
    1657                 :            : #endif
    

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