Branch data     Line data    Source code

       1                 :            : // SPDX-License-Identifier: GPL-2.0-only
       2                 :            : #define pr_fmt(fmt) "%s: " fmt "\n", __func__
       3                 :            : 
       4                 :            : #include <linux/kernel.h>
       5                 :            : #include <linux/sched.h>
       6                 :            : #include <linux/wait.h>
       7                 :            : #include <linux/percpu-refcount.h>
       8                 :            : 
       9                 :            : /*
      10                 :            :  * Initially, a percpu refcount is just a set of percpu counters. Initially, we
      11                 :            :  * don't try to detect the ref hitting 0 - which means that get/put can just
      12                 :            :  * increment or decrement the local counter. Note that the counter on a
      13                 :            :  * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
      14                 :            :  * percpu counters will all sum to the correct value
      15                 :            :  *
      16                 :            :  * (More precisely: because modular arithmetic is commutative the sum of all the
      17                 :            :  * percpu_count vars will be equal to what it would have been if all the gets
      18                 :            :  * and puts were done to a single integer, even if some of the percpu integers
      19                 :            :  * overflow or underflow).
      20                 :            :  *
      21                 :            :  * The real trick to implementing percpu refcounts is shutdown. We can't detect
      22                 :            :  * the ref hitting 0 on every put - this would require global synchronization
      23                 :            :  * and defeat the whole purpose of using percpu refs.
      24                 :            :  *
      25                 :            :  * What we do is require the user to keep track of the initial refcount; we know
      26                 :            :  * the ref can't hit 0 before the user drops the initial ref, so as long as we
      27                 :            :  * convert to non percpu mode before the initial ref is dropped everything
      28                 :            :  * works.
      29                 :            :  *
      30                 :            :  * Converting to non percpu mode is done with some RCUish stuff in
      31                 :            :  * percpu_ref_kill. Additionally, we need a bias value so that the
      32                 :            :  * atomic_long_t can't hit 0 before we've added up all the percpu refs.
      33                 :            :  */
      34                 :            : 
      35                 :            : #define PERCPU_COUNT_BIAS       (1LU << (BITS_PER_LONG - 1))
      36                 :            : 
      37                 :            : static DEFINE_SPINLOCK(percpu_ref_switch_lock);
      38                 :            : static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
      39                 :            : 
      40                 :            : static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
      41                 :            : {
      42                 :          3 :         return (unsigned long __percpu *)
      43                 :          3 :                 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
      44                 :            : }
      45                 :            : 
      46                 :            : /**
      47                 :            :  * percpu_ref_init - initialize a percpu refcount
      48                 :            :  * @ref: percpu_ref to initialize
      49                 :            :  * @release: function which will be called when refcount hits 0
      50                 :            :  * @flags: PERCPU_REF_INIT_* flags
      51                 :            :  * @gfp: allocation mask to use
      52                 :            :  *
      53                 :            :  * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a
      54                 :            :  * refcount of 1; analagous to atomic_long_set(ref, 1).  See the
      55                 :            :  * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
      56                 :            :  *
      57                 :            :  * Note that @release must not sleep - it may potentially be called from RCU
      58                 :            :  * callback context by percpu_ref_kill().
      59                 :            :  */
      60                 :          3 : int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
      61                 :            :                     unsigned int flags, gfp_t gfp)
      62                 :            : {
      63                 :            :         size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
      64                 :            :                              __alignof__(unsigned long));
      65                 :            :         unsigned long start_count = 0;
      66                 :            : 
      67                 :          3 :         ref->percpu_count_ptr = (unsigned long)
      68                 :          3 :                 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
      69                 :          3 :         if (!ref->percpu_count_ptr)
      70                 :            :                 return -ENOMEM;
      71                 :            : 
      72                 :          3 :         ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
      73                 :          3 :         ref->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
      74                 :            : 
      75                 :          3 :         if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
      76                 :          3 :                 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
      77                 :          3 :                 ref->allow_reinit = true;
      78                 :            :         } else {
      79                 :            :                 start_count += PERCPU_COUNT_BIAS;
      80                 :            :         }
      81                 :            : 
      82                 :          3 :         if (flags & PERCPU_REF_INIT_DEAD)
      83                 :          0 :                 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
      84                 :            :         else
      85                 :          3 :                 start_count++;
      86                 :            : 
      87                 :            :         atomic_long_set(&ref->count, start_count);
      88                 :            : 
      89                 :          3 :         ref->release = release;
      90                 :          3 :         ref->confirm_switch = NULL;
      91                 :          3 :         return 0;
      92                 :            : }
      93                 :            : EXPORT_SYMBOL_GPL(percpu_ref_init);
      94                 :            : 
      95                 :            : /**
      96                 :            :  * percpu_ref_exit - undo percpu_ref_init()
      97                 :            :  * @ref: percpu_ref to exit
      98                 :            :  *
      99                 :            :  * This function exits @ref.  The caller is responsible for ensuring that
     100                 :            :  * @ref is no longer in active use.  The usual places to invoke this
     101                 :            :  * function from are the @ref->release() callback or in init failure path
     102                 :            :  * where percpu_ref_init() succeeded but other parts of the initialization
     103                 :            :  * of the embedding object failed.
     104                 :            :  */
     105                 :          3 : void percpu_ref_exit(struct percpu_ref *ref)
     106                 :            : {
     107                 :            :         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
     108                 :            : 
     109                 :          3 :         if (percpu_count) {
     110                 :            :                 /* non-NULL confirm_switch indicates switching in progress */
     111                 :          3 :                 WARN_ON_ONCE(ref->confirm_switch);
     112                 :          3 :                 free_percpu(percpu_count);
     113                 :          3 :                 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
     114                 :            :         }
     115                 :          3 : }
     116                 :            : EXPORT_SYMBOL_GPL(percpu_ref_exit);
     117                 :            : 
     118                 :          3 : static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
     119                 :            : {
     120                 :          3 :         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
     121                 :            : 
     122                 :          3 :         ref->confirm_switch(ref);
     123                 :          3 :         ref->confirm_switch = NULL;
     124                 :          3 :         wake_up_all(&percpu_ref_switch_waitq);
     125                 :            : 
     126                 :          3 :         if (!ref->allow_reinit)
     127                 :          3 :                 percpu_ref_exit(ref);
     128                 :            : 
     129                 :            :         /* drop ref from percpu_ref_switch_to_atomic() */
     130                 :            :         percpu_ref_put(ref);
     131                 :          3 : }
     132                 :            : 
     133                 :          3 : static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
     134                 :            : {
     135                 :            :         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
     136                 :            :         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
     137                 :            :         unsigned long count = 0;
     138                 :            :         int cpu;
     139                 :            : 
     140                 :          3 :         for_each_possible_cpu(cpu)
     141                 :          3 :                 count += *per_cpu_ptr(percpu_count, cpu);
     142                 :            : 
     143                 :            :         pr_debug("global %ld percpu %ld",
     144                 :            :                  atomic_long_read(&ref->count), (long)count);
     145                 :            : 
     146                 :            :         /*
     147                 :            :          * It's crucial that we sum the percpu counters _before_ adding the sum
     148                 :            :          * to &ref->count; since gets could be happening on one cpu while puts
     149                 :            :          * happen on another, adding a single cpu's count could cause
     150                 :            :          * @ref->count to hit 0 before we've got a consistent value - but the
     151                 :            :          * sum of all the counts will be consistent and correct.
     152                 :            :          *
     153                 :            :          * Subtracting the bias value then has to happen _after_ adding count to
     154                 :            :          * &ref->count; we need the bias value to prevent &ref->count from
     155                 :            :          * reaching 0 before we add the percpu counts. But doing it at the same
     156                 :            :          * time is equivalent and saves us atomic operations:
     157                 :            :          */
     158                 :          3 :         atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
     159                 :            : 
     160                 :          3 :         WARN_ONCE(atomic_long_read(&ref->count) <= 0,
     161                 :            :                   "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
     162                 :            :                   ref->release, atomic_long_read(&ref->count));
     163                 :            : 
     164                 :            :         /* @ref is viewed as dead on all CPUs, send out switch confirmation */
     165                 :          3 :         percpu_ref_call_confirm_rcu(rcu);
     166                 :          3 : }
     167                 :            : 
     168                 :          3 : static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
     169                 :            : {
     170                 :          3 : }
     171                 :            : 
     172                 :          3 : static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
     173                 :            :                                           percpu_ref_func_t *confirm_switch)
     174                 :            : {
     175                 :          3 :         if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {
     176                 :          3 :                 if (confirm_switch)
     177                 :          0 :                         confirm_switch(ref);
     178                 :          3 :                 return;
     179                 :            :         }
     180                 :            : 
     181                 :            :         /* switching from percpu to atomic */
     182                 :          3 :         ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
     183                 :            : 
     184                 :            :         /*
     185                 :            :          * Non-NULL ->confirm_switch is used to indicate that switching is
     186                 :            :          * in progress.  Use noop one if unspecified.
     187                 :            :          */
     188                 :          3 :         ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch;
     189                 :            : 
     190                 :            :         percpu_ref_get(ref);    /* put after confirmation */
     191                 :          3 :         call_rcu(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
     192                 :            : }
     193                 :            : 
     194                 :          3 : static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
     195                 :            : {
     196                 :            :         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
     197                 :            :         int cpu;
     198                 :            : 
     199                 :          3 :         BUG_ON(!percpu_count);
     200                 :            : 
     201                 :          3 :         if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
     202                 :            :                 return;
     203                 :            : 
     204                 :          3 :         if (WARN_ON_ONCE(!ref->allow_reinit))
     205                 :            :                 return;
     206                 :            : 
     207                 :          3 :         atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
     208                 :            : 
     209                 :            :         /*
     210                 :            :          * Restore per-cpu operation.  smp_store_release() is paired
     211                 :            :          * with READ_ONCE() in __ref_is_percpu() and guarantees that the
     212                 :            :          * zeroing is visible to all percpu accesses which can see the
     213                 :            :          * following __PERCPU_REF_ATOMIC clearing.
     214                 :            :          */
     215                 :          3 :         for_each_possible_cpu(cpu)
     216                 :          3 :                 *per_cpu_ptr(percpu_count, cpu) = 0;
     217                 :            : 
     218                 :          3 :         smp_store_release(&ref->percpu_count_ptr,
     219                 :            :                           ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
     220                 :            : }
     221                 :            : 
     222                 :          3 : static void __percpu_ref_switch_mode(struct percpu_ref *ref,
     223                 :            :                                      percpu_ref_func_t *confirm_switch)
     224                 :            : {
     225                 :            :         lockdep_assert_held(&percpu_ref_switch_lock);
     226                 :            : 
     227                 :            :         /*
     228                 :            :          * If the previous ATOMIC switching hasn't finished yet, wait for
     229                 :            :          * its completion.  If the caller ensures that ATOMIC switching
     230                 :            :          * isn't in progress, this function can be called from any context.
     231                 :            :          */
     232                 :          3 :         wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,
     233                 :            :                             percpu_ref_switch_lock);
     234                 :            : 
     235                 :          3 :         if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
     236                 :          3 :                 __percpu_ref_switch_to_atomic(ref, confirm_switch);
     237                 :            :         else
     238                 :          3 :                 __percpu_ref_switch_to_percpu(ref);
     239                 :          3 : }
     240                 :            : 
     241                 :            : /**
     242                 :            :  * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
     243                 :            :  * @ref: percpu_ref to switch to atomic mode
     244                 :            :  * @confirm_switch: optional confirmation callback
     245                 :            :  *
     246                 :            :  * There's no reason to use this function for the usual reference counting.
     247                 :            :  * Use percpu_ref_kill[_and_confirm]().
     248                 :            :  *
     249                 :            :  * Schedule switching of @ref to atomic mode.  All its percpu counts will
     250                 :            :  * be collected to the main atomic counter.  On completion, when all CPUs
     251                 :            :  * are guaraneed to be in atomic mode, @confirm_switch, which may not
     252                 :            :  * block, is invoked.  This function may be invoked concurrently with all
     253                 :            :  * the get/put operations and can safely be mixed with kill and reinit
     254                 :            :  * operations.  Note that @ref will stay in atomic mode across kill/reinit
     255                 :            :  * cycles until percpu_ref_switch_to_percpu() is called.
     256                 :            :  *
     257                 :            :  * This function may block if @ref is in the process of switching to atomic
     258                 :            :  * mode.  If the caller ensures that @ref is not in the process of
     259                 :            :  * switching to atomic mode, this function can be called from any context.
     260                 :            :  */
     261                 :          0 : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
     262                 :            :                                  percpu_ref_func_t *confirm_switch)
     263                 :            : {
     264                 :            :         unsigned long flags;
     265                 :            : 
     266                 :          0 :         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
     267                 :            : 
     268                 :          0 :         ref->force_atomic = true;
     269                 :          0 :         __percpu_ref_switch_mode(ref, confirm_switch);
     270                 :            : 
     271                 :            :         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
     272                 :          0 : }
     273                 :            : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
     274                 :            : 
     275                 :            : /**
     276                 :            :  * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
     277                 :            :  * @ref: percpu_ref to switch to atomic mode
     278                 :            :  *
     279                 :            :  * Schedule switching the ref to atomic mode, and wait for the
     280                 :            :  * switch to complete.  Caller must ensure that no other thread
     281                 :            :  * will switch back to percpu mode.
     282                 :            :  */
     283                 :          0 : void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
     284                 :            : {
     285                 :          0 :         percpu_ref_switch_to_atomic(ref, NULL);
     286                 :          0 :         wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
     287                 :          0 : }
     288                 :            : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
     289                 :            : 
     290                 :            : /**
     291                 :            :  * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
     292                 :            :  * @ref: percpu_ref to switch to percpu mode
     293                 :            :  *
     294                 :            :  * There's no reason to use this function for the usual reference counting.
     295                 :            :  * To re-use an expired ref, use percpu_ref_reinit().
     296                 :            :  *
     297                 :            :  * Switch @ref to percpu mode.  This function may be invoked concurrently
     298                 :            :  * with all the get/put operations and can safely be mixed with kill and
     299                 :            :  * reinit operations.  This function reverses the sticky atomic state set
     300                 :            :  * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is
     301                 :            :  * dying or dead, the actual switching takes place on the following
     302                 :            :  * percpu_ref_reinit().
     303                 :            :  *
     304                 :            :  * This function may block if @ref is in the process of switching to atomic
     305                 :            :  * mode.  If the caller ensures that @ref is not in the process of
     306                 :            :  * switching to atomic mode, this function can be called from any context.
     307                 :            :  */
     308                 :          3 : void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
     309                 :            : {
     310                 :            :         unsigned long flags;
     311                 :            : 
     312                 :          3 :         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
     313                 :            : 
     314                 :          3 :         ref->force_atomic = false;
     315                 :          3 :         __percpu_ref_switch_mode(ref, NULL);
     316                 :            : 
     317                 :            :         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
     318                 :          3 : }
     319                 :            : EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
     320                 :            : 
     321                 :            : /**
     322                 :            :  * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
     323                 :            :  * @ref: percpu_ref to kill
     324                 :            :  * @confirm_kill: optional confirmation callback
     325                 :            :  *
     326                 :            :  * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
     327                 :            :  * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
     328                 :            :  * called after @ref is seen as dead from all CPUs at which point all
     329                 :            :  * further invocations of percpu_ref_tryget_live() will fail.  See
     330                 :            :  * percpu_ref_tryget_live() for details.
     331                 :            :  *
     332                 :            :  * This function normally doesn't block and can be called from any context
     333                 :            :  * but it may block if @confirm_kill is specified and @ref is in the
     334                 :            :  * process of switching to atomic mode by percpu_ref_switch_to_atomic().
     335                 :            :  *
     336                 :            :  * There are no implied RCU grace periods between kill and release.
     337                 :            :  */
     338                 :          3 : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
     339                 :            :                                  percpu_ref_func_t *confirm_kill)
     340                 :            : {
     341                 :            :         unsigned long flags;
     342                 :            : 
     343                 :          3 :         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
     344                 :            : 
     345                 :          3 :         WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
     346                 :            :                   "%s called more than once on %ps!", __func__, ref->release);
     347                 :            : 
     348                 :          3 :         ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
     349                 :          3 :         __percpu_ref_switch_mode(ref, confirm_kill);
     350                 :            :         percpu_ref_put(ref);
     351                 :            : 
     352                 :            :         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
     353                 :          3 : }
     354                 :            : EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
     355                 :            : 
     356                 :            : /**
     357                 :            :  * percpu_ref_reinit - re-initialize a percpu refcount
     358                 :            :  * @ref: perpcu_ref to re-initialize
     359                 :            :  *
     360                 :            :  * Re-initialize @ref so that it's in the same state as when it finished
     361                 :            :  * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been
     362                 :            :  * initialized successfully and reached 0 but not exited.
     363                 :            :  *
     364                 :            :  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
     365                 :            :  * this function is in progress.
     366                 :            :  */
     367                 :          0 : void percpu_ref_reinit(struct percpu_ref *ref)
     368                 :            : {
     369                 :          0 :         WARN_ON_ONCE(!percpu_ref_is_zero(ref));
     370                 :            : 
     371                 :          0 :         percpu_ref_resurrect(ref);
     372                 :          0 : }
     373                 :            : EXPORT_SYMBOL_GPL(percpu_ref_reinit);
     374                 :            : 
     375                 :            : /**
     376                 :            :  * percpu_ref_resurrect - modify a percpu refcount from dead to live
     377                 :            :  * @ref: perpcu_ref to resurrect
     378                 :            :  *
     379                 :            :  * Modify @ref so that it's in the same state as before percpu_ref_kill() was
     380                 :            :  * called. @ref must be dead but must not yet have exited.
     381                 :            :  *
     382                 :            :  * If @ref->release() frees @ref then the caller is responsible for
     383                 :            :  * guaranteeing that @ref->release() does not get called while this
     384                 :            :  * function is in progress.
     385                 :            :  *
     386                 :            :  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
     387                 :            :  * this function is in progress.
     388                 :            :  */
     389                 :          3 : void percpu_ref_resurrect(struct percpu_ref *ref)
     390                 :            : {
     391                 :            :         unsigned long __percpu *percpu_count;
     392                 :            :         unsigned long flags;
     393                 :            : 
     394                 :          3 :         spin_lock_irqsave(&percpu_ref_switch_lock, flags);
     395                 :            : 
     396                 :          3 :         WARN_ON_ONCE(!(ref->percpu_count_ptr & __PERCPU_REF_DEAD));
     397                 :          3 :         WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
     398                 :            : 
     399                 :          3 :         ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
     400                 :            :         percpu_ref_get(ref);
     401                 :          3 :         __percpu_ref_switch_mode(ref, NULL);
     402                 :            : 
     403                 :            :         spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
     404                 :          3 : }
     405                 :            : EXPORT_SYMBOL_GPL(percpu_ref_resurrect);