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       1                 :            : /* SPDX-License-Identifier: GPL-2.0 */
       2                 :            : /*
       3                 :            :  * Percpu refcounts:
       4                 :            :  * (C) 2012 Google, Inc.
       5                 :            :  * Author: Kent Overstreet <koverstreet@google.com>
       6                 :            :  *
       7                 :            :  * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
       8                 :            :  * atomic_dec_and_test() - but percpu.
       9                 :            :  *
      10                 :            :  * There's one important difference between percpu refs and normal atomic_t
      11                 :            :  * refcounts; you have to keep track of your initial refcount, and then when you
      12                 :            :  * start shutting down you call percpu_ref_kill() _before_ dropping the initial
      13                 :            :  * refcount.
      14                 :            :  *
      15                 :            :  * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
      16                 :            :  * than an atomic_t - this is because of the way shutdown works, see
      17                 :            :  * percpu_ref_kill()/PERCPU_COUNT_BIAS.
      18                 :            :  *
      19                 :            :  * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
      20                 :            :  * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
      21                 :            :  * puts the ref back in single atomic_t mode, collecting the per cpu refs and
      22                 :            :  * issuing the appropriate barriers, and then marks the ref as shutting down so
      23                 :            :  * that percpu_ref_put() will check for the ref hitting 0.  After it returns,
      24                 :            :  * it's safe to drop the initial ref.
      25                 :            :  *
      26                 :            :  * USAGE:
      27                 :            :  *
      28                 :            :  * See fs/aio.c for some example usage; it's used there for struct kioctx, which
      29                 :            :  * is created when userspaces calls io_setup(), and destroyed when userspace
      30                 :            :  * calls io_destroy() or the process exits.
      31                 :            :  *
      32                 :            :  * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
      33                 :            :  * removes the kioctx from the proccess's table of kioctxs and kills percpu_ref.
      34                 :            :  * After that, there can't be any new users of the kioctx (from lookup_ioctx())
      35                 :            :  * and it's then safe to drop the initial ref with percpu_ref_put().
      36                 :            :  *
      37                 :            :  * Note that the free path, free_ioctx(), needs to go through explicit call_rcu()
      38                 :            :  * to synchronize with RCU protected lookup_ioctx().  percpu_ref operations don't
      39                 :            :  * imply RCU grace periods of any kind and if a user wants to combine percpu_ref
      40                 :            :  * with RCU protection, it must be done explicitly.
      41                 :            :  *
      42                 :            :  * Code that does a two stage shutdown like this often needs some kind of
      43                 :            :  * explicit synchronization to ensure the initial refcount can only be dropped
      44                 :            :  * once - percpu_ref_kill() does this for you, it returns true once and false if
      45                 :            :  * someone else already called it. The aio code uses it this way, but it's not
      46                 :            :  * necessary if the code has some other mechanism to synchronize teardown.
      47                 :            :  * around.
      48                 :            :  */
      49                 :            : 
      50                 :            : #ifndef _LINUX_PERCPU_REFCOUNT_H
      51                 :            : #define _LINUX_PERCPU_REFCOUNT_H
      52                 :            : 
      53                 :            : #include <linux/atomic.h>
      54                 :            : #include <linux/kernel.h>
      55                 :            : #include <linux/percpu.h>
      56                 :            : #include <linux/rcupdate.h>
      57                 :            : #include <linux/gfp.h>
      58                 :            : 
      59                 :            : struct percpu_ref;
      60                 :            : typedef void (percpu_ref_func_t)(struct percpu_ref *);
      61                 :            : 
      62                 :            : /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
      63                 :            : enum {
      64                 :            :         __PERCPU_REF_ATOMIC     = 1LU << 0,       /* operating in atomic mode */
      65                 :            :         __PERCPU_REF_DEAD       = 1LU << 1,       /* (being) killed */
      66                 :            :         __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
      67                 :            : 
      68                 :            :         __PERCPU_REF_FLAG_BITS  = 2,
      69                 :            : };
      70                 :            : 
      71                 :            : /* @flags for percpu_ref_init() */
      72                 :            : enum {
      73                 :            :         /*
      74                 :            :          * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
      75                 :            :          * operation using percpu_ref_switch_to_percpu().  If initialized
      76                 :            :          * with this flag, the ref will stay in atomic mode until
      77                 :            :          * percpu_ref_switch_to_percpu() is invoked on it.
      78                 :            :          * Implies ALLOW_REINIT.
      79                 :            :          */
      80                 :            :         PERCPU_REF_INIT_ATOMIC  = 1 << 0,
      81                 :            : 
      82                 :            :         /*
      83                 :            :          * Start dead w/ ref == 0 in atomic mode.  Must be revived with
      84                 :            :          * percpu_ref_reinit() before used.  Implies INIT_ATOMIC and
      85                 :            :          * ALLOW_REINIT.
      86                 :            :          */
      87                 :            :         PERCPU_REF_INIT_DEAD    = 1 << 1,
      88                 :            : 
      89                 :            :         /*
      90                 :            :          * Allow switching from atomic mode to percpu mode.
      91                 :            :          */
      92                 :            :         PERCPU_REF_ALLOW_REINIT = 1 << 2,
      93                 :            : };
      94                 :            : 
      95                 :            : struct percpu_ref {
      96                 :            :         atomic_long_t           count;
      97                 :            :         /*
      98                 :            :          * The low bit of the pointer indicates whether the ref is in percpu
      99                 :            :          * mode; if set, then get/put will manipulate the atomic_t.
     100                 :            :          */
     101                 :            :         unsigned long           percpu_count_ptr;
     102                 :            :         percpu_ref_func_t       *release;
     103                 :            :         percpu_ref_func_t       *confirm_switch;
     104                 :            :         bool                    force_atomic:1;
     105                 :            :         bool                    allow_reinit:1;
     106                 :            :         struct rcu_head         rcu;
     107                 :            : };
     108                 :            : 
     109                 :            : int __must_check percpu_ref_init(struct percpu_ref *ref,
     110                 :            :                                  percpu_ref_func_t *release, unsigned int flags,
     111                 :            :                                  gfp_t gfp);
     112                 :            : void percpu_ref_exit(struct percpu_ref *ref);
     113                 :            : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
     114                 :            :                                  percpu_ref_func_t *confirm_switch);
     115                 :            : void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
     116                 :            : void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
     117                 :            : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
     118                 :            :                                  percpu_ref_func_t *confirm_kill);
     119                 :            : void percpu_ref_resurrect(struct percpu_ref *ref);
     120                 :            : void percpu_ref_reinit(struct percpu_ref *ref);
     121                 :            : 
     122                 :            : /**
     123                 :            :  * percpu_ref_kill - drop the initial ref
     124                 :            :  * @ref: percpu_ref to kill
     125                 :            :  *
     126                 :            :  * Must be used to drop the initial ref on a percpu refcount; must be called
     127                 :            :  * precisely once before shutdown.
     128                 :            :  *
     129                 :            :  * Switches @ref into atomic mode before gathering up the percpu counters
     130                 :            :  * and dropping the initial ref.
     131                 :            :  *
     132                 :            :  * There are no implied RCU grace periods between kill and release.
     133                 :            :  */
     134                 :            : static inline void percpu_ref_kill(struct percpu_ref *ref)
     135                 :            : {
     136                 :          3 :         percpu_ref_kill_and_confirm(ref, NULL);
     137                 :            : }
     138                 :            : 
     139                 :            : /*
     140                 :            :  * Internal helper.  Don't use outside percpu-refcount proper.  The
     141                 :            :  * function doesn't return the pointer and let the caller test it for NULL
     142                 :            :  * because doing so forces the compiler to generate two conditional
     143                 :            :  * branches as it can't assume that @ref->percpu_count is not NULL.
     144                 :            :  */
     145                 :            : static inline bool __ref_is_percpu(struct percpu_ref *ref,
     146                 :            :                                           unsigned long __percpu **percpu_countp)
     147                 :            : {
     148                 :            :         unsigned long percpu_ptr;
     149                 :            : 
     150                 :            :         /*
     151                 :            :          * The value of @ref->percpu_count_ptr is tested for
     152                 :            :          * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
     153                 :            :          * used as a pointer.  If the compiler generates a separate fetch
     154                 :            :          * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
     155                 :            :          * between contaminating the pointer value, meaning that
     156                 :            :          * READ_ONCE() is required when fetching it.
     157                 :            :          *
     158                 :            :          * The smp_read_barrier_depends() implied by READ_ONCE() pairs
     159                 :            :          * with smp_store_release() in __percpu_ref_switch_to_percpu().
     160                 :            :          */
     161                 :            :         percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
     162                 :            : 
     163                 :            :         /*
     164                 :            :          * Theoretically, the following could test just ATOMIC; however,
     165                 :            :          * then we'd have to mask off DEAD separately as DEAD may be
     166                 :            :          * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
     167                 :            :          * implies ATOMIC anyway.  Test them together.
     168                 :            :          */
     169                 :          3 :         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
     170                 :            :                 return false;
     171                 :            : 
     172                 :          3 :         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
     173                 :            :         return true;
     174                 :            : }
     175                 :            : 
     176                 :            : /**
     177                 :            :  * percpu_ref_get_many - increment a percpu refcount
     178                 :            :  * @ref: percpu_ref to get
     179                 :            :  * @nr: number of references to get
     180                 :            :  *
     181                 :            :  * Analogous to atomic_long_add().
     182                 :            :  *
     183                 :            :  * This function is safe to call as long as @ref is between init and exit.
     184                 :            :  */
     185                 :          3 : static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
     186                 :            : {
     187                 :            :         unsigned long __percpu *percpu_count;
     188                 :            : 
     189                 :            :         rcu_read_lock_sched();
     190                 :            : 
     191                 :          3 :         if (__ref_is_percpu(ref, &percpu_count))
     192                 :          3 :                 this_cpu_add(*percpu_count, nr);
     193                 :            :         else
     194                 :          3 :                 atomic_long_add(nr, &ref->count);
     195                 :            : 
     196                 :            :         rcu_read_unlock_sched();
     197                 :          3 : }
     198                 :            : 
     199                 :            : /**
     200                 :            :  * percpu_ref_get - increment a percpu refcount
     201                 :            :  * @ref: percpu_ref to get
     202                 :            :  *
     203                 :            :  * Analagous to atomic_long_inc().
     204                 :            :  *
     205                 :            :  * This function is safe to call as long as @ref is between init and exit.
     206                 :            :  */
     207                 :            : static inline void percpu_ref_get(struct percpu_ref *ref)
     208                 :            : {
     209                 :          3 :         percpu_ref_get_many(ref, 1);
     210                 :            : }
     211                 :            : 
     212                 :            : /**
     213                 :            :  * percpu_ref_tryget - try to increment a percpu refcount
     214                 :            :  * @ref: percpu_ref to try-get
     215                 :            :  *
     216                 :            :  * Increment a percpu refcount unless its count already reached zero.
     217                 :            :  * Returns %true on success; %false on failure.
     218                 :            :  *
     219                 :            :  * This function is safe to call as long as @ref is between init and exit.
     220                 :            :  */
     221                 :          3 : static inline bool percpu_ref_tryget(struct percpu_ref *ref)
     222                 :            : {
     223                 :            :         unsigned long __percpu *percpu_count;
     224                 :            :         bool ret;
     225                 :            : 
     226                 :            :         rcu_read_lock_sched();
     227                 :            : 
     228                 :          3 :         if (__ref_is_percpu(ref, &percpu_count)) {
     229                 :          3 :                 this_cpu_inc(*percpu_count);
     230                 :            :                 ret = true;
     231                 :            :         } else {
     232                 :          0 :                 ret = atomic_long_inc_not_zero(&ref->count);
     233                 :            :         }
     234                 :            : 
     235                 :            :         rcu_read_unlock_sched();
     236                 :            : 
     237                 :          3 :         return ret;
     238                 :            : }
     239                 :            : 
     240                 :            : /**
     241                 :            :  * percpu_ref_tryget_live - try to increment a live percpu refcount
     242                 :            :  * @ref: percpu_ref to try-get
     243                 :            :  *
     244                 :            :  * Increment a percpu refcount unless it has already been killed.  Returns
     245                 :            :  * %true on success; %false on failure.
     246                 :            :  *
     247                 :            :  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
     248                 :            :  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
     249                 :            :  * should be used.  After the confirm_kill callback is invoked, it's
     250                 :            :  * guaranteed that no new reference will be given out by
     251                 :            :  * percpu_ref_tryget_live().
     252                 :            :  *
     253                 :            :  * This function is safe to call as long as @ref is between init and exit.
     254                 :            :  */
     255                 :          3 : static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
     256                 :            : {
     257                 :            :         unsigned long __percpu *percpu_count;
     258                 :            :         bool ret = false;
     259                 :            : 
     260                 :            :         rcu_read_lock_sched();
     261                 :            : 
     262                 :          3 :         if (__ref_is_percpu(ref, &percpu_count)) {
     263                 :          3 :                 this_cpu_inc(*percpu_count);
     264                 :            :                 ret = true;
     265                 :          3 :         } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
     266                 :          3 :                 ret = atomic_long_inc_not_zero(&ref->count);
     267                 :            :         }
     268                 :            : 
     269                 :            :         rcu_read_unlock_sched();
     270                 :            : 
     271                 :          3 :         return ret;
     272                 :            : }
     273                 :            : 
     274                 :            : /**
     275                 :            :  * percpu_ref_put_many - decrement a percpu refcount
     276                 :            :  * @ref: percpu_ref to put
     277                 :            :  * @nr: number of references to put
     278                 :            :  *
     279                 :            :  * Decrement the refcount, and if 0, call the release function (which was passed
     280                 :            :  * to percpu_ref_init())
     281                 :            :  *
     282                 :            :  * This function is safe to call as long as @ref is between init and exit.
     283                 :            :  */
     284                 :          3 : static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
     285                 :            : {
     286                 :            :         unsigned long __percpu *percpu_count;
     287                 :            : 
     288                 :            :         rcu_read_lock_sched();
     289                 :            : 
     290                 :          3 :         if (__ref_is_percpu(ref, &percpu_count))
     291                 :          3 :                 this_cpu_sub(*percpu_count, nr);
     292                 :          3 :         else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
     293                 :          3 :                 ref->release(ref);
     294                 :            : 
     295                 :            :         rcu_read_unlock_sched();
     296                 :          3 : }
     297                 :            : 
     298                 :            : /**
     299                 :            :  * percpu_ref_put - decrement a percpu refcount
     300                 :            :  * @ref: percpu_ref to put
     301                 :            :  *
     302                 :            :  * Decrement the refcount, and if 0, call the release function (which was passed
     303                 :            :  * to percpu_ref_init())
     304                 :            :  *
     305                 :            :  * This function is safe to call as long as @ref is between init and exit.
     306                 :            :  */
     307                 :            : static inline void percpu_ref_put(struct percpu_ref *ref)
     308                 :            : {
     309                 :          3 :         percpu_ref_put_many(ref, 1);
     310                 :            : }
     311                 :            : 
     312                 :            : /**
     313                 :            :  * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
     314                 :            :  * @ref: percpu_ref to test
     315                 :            :  *
     316                 :            :  * Returns %true if @ref is dying or dead.
     317                 :            :  *
     318                 :            :  * This function is safe to call as long as @ref is between init and exit
     319                 :            :  * and the caller is responsible for synchronizing against state changes.
     320                 :            :  */
     321                 :            : static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
     322                 :            : {
     323                 :          3 :         return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
     324                 :            : }
     325                 :            : 
     326                 :            : /**
     327                 :            :  * percpu_ref_is_zero - test whether a percpu refcount reached zero
     328                 :            :  * @ref: percpu_ref to test
     329                 :            :  *
     330                 :            :  * Returns %true if @ref reached zero.
     331                 :            :  *
     332                 :            :  * This function is safe to call as long as @ref is between init and exit.
     333                 :            :  */
     334                 :            : static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
     335                 :            : {
     336                 :            :         unsigned long __percpu *percpu_count;
     337                 :            : 
     338                 :          3 :         if (__ref_is_percpu(ref, &percpu_count))
     339                 :            :                 return false;
     340                 :          3 :         return !atomic_long_read(&ref->count);
     341                 :            : }
     342                 :            : 
     343                 :            : #endif