include/linux/percpu-refcount.h

   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          */
  79         PERCPU_REF_INIT_ATOMIC  = 1 << 0,
  80
  81         /*
  82          * Start dead w/ ref == 0 in atomic mode.  Must be revived with
  83          * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
  84          */
  85         PERCPU_REF_INIT_DEAD    = 1 << 1,
  86 };
  87
  88 struct percpu_ref {
  89         atomic_long_t           count;
  90         /*
  91          * The low bit of the pointer indicates whether the ref is in percpu
  92          * mode; if set, then get/put will manipulate the atomic_t.
  93          */
  94         unsigned long           percpu_count_ptr;
  95         percpu_ref_func_t       *release;
  96         percpu_ref_func_t       *confirm_switch;
  97         bool                    force_atomic:1;
  98         struct rcu_head         rcu;
  99 };
 100
 101 int __must_check percpu_ref_init(struct percpu_ref *ref,
 102                                  percpu_ref_func_t *release, unsigned int flags,
 103                                  gfp_t gfp);
 104 void percpu_ref_exit(struct percpu_ref *ref);
 105 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 106                                  percpu_ref_func_t *confirm_switch);
 107 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
 108 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
 109 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 110                                  percpu_ref_func_t *confirm_kill);
 111 void percpu_ref_resurrect(struct percpu_ref *ref);
 112 void percpu_ref_reinit(struct percpu_ref *ref);
 113
 114 /**
 115  * percpu_ref_kill - drop the initial ref
 116  * @ref: percpu_ref to kill
 117  *
 118  * Must be used to drop the initial ref on a percpu refcount; must be called
 119  * precisely once before shutdown.
 120  *
 121  * Switches @ref into atomic mode before gathering up the percpu counters
 122  * and dropping the initial ref.
 123  *
 124  * There are no implied RCU grace periods between kill and release.
 125  */
 126 static inline void percpu_ref_kill(struct percpu_ref *ref)
 127 {
 128         percpu_ref_kill_and_confirm(ref, NULL);
 129 }
 130
 131 /*
 132  * Internal helper.  Don't use outside percpu-refcount proper.  The
 133  * function doesn't return the pointer and let the caller test it for NULL
 134  * because doing so forces the compiler to generate two conditional
 135  * branches as it can't assume that @ref->percpu_count is not NULL.
 136  */
 137 static inline bool __ref_is_percpu(struct percpu_ref *ref,
 138                                           unsigned long __percpu **percpu_countp)
 139 {
 140         unsigned long percpu_ptr;
 141
 142         /*
 143          * The value of @ref->percpu_count_ptr is tested for
 144          * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
 145          * used as a pointer.  If the compiler generates a separate fetch
 146          * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
 147          * between contaminating the pointer value, meaning that
 148          * READ_ONCE() is required when fetching it.
 149          *
 150          * The smp_read_barrier_depends() implied by READ_ONCE() pairs
 151          * with smp_store_release() in __percpu_ref_switch_to_percpu().
 152          */
 153         percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
 154
 155         /*
 156          * Theoretically, the following could test just ATOMIC; however,
 157          * then we'd have to mask off DEAD separately as DEAD may be
 158          * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
 159          * implies ATOMIC anyway.  Test them together.
 160          */
 161         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
 162                 return false;
 163
 164         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
 165         return true;
 166 }
 167
 168 /**
 169  * percpu_ref_get_many - increment a percpu refcount
 170  * @ref: percpu_ref to get
 171  * @nr: number of references to get
 172  *
 173  * Analogous to atomic_long_add().
 174  *
 175  * This function is safe to call as long as @ref is between init and exit.
 176  */
 177 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
 178 {
 179         unsigned long __percpu *percpu_count;
 180
 181         rcu_read_lock_sched();
 182
 183         if (__ref_is_percpu(ref, &percpu_count))
 184                 this_cpu_add(*percpu_count, nr);
 185         else
 186                 atomic_long_add(nr, &ref->count);
 187
 188         rcu_read_unlock_sched();
 189 }
 190
 191 /**
 192  * percpu_ref_get - increment a percpu refcount
 193  * @ref: percpu_ref to get
 194  *
 195  * Analagous to atomic_long_inc().
 196  *
 197  * This function is safe to call as long as @ref is between init and exit.
 198  */
 199 static inline void percpu_ref_get(struct percpu_ref *ref)
 200 {
 201         percpu_ref_get_many(ref, 1);
 202 }
 203
 204 /**
 205  * percpu_ref_tryget - try to increment a percpu refcount
 206  * @ref: percpu_ref to try-get
 207  *
 208  * Increment a percpu refcount unless its count already reached zero.
 209  * Returns %true on success; %false on failure.
 210  *
 211  * This function is safe to call as long as @ref is between init and exit.
 212  */
 213 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 214 {
 215         unsigned long __percpu *percpu_count;
 216         bool ret;
 217
 218         rcu_read_lock_sched();
 219
 220         if (__ref_is_percpu(ref, &percpu_count)) {
 221                 this_cpu_inc(*percpu_count);
 222                 ret = true;
 223         } else {
 224                 ret = atomic_long_inc_not_zero(&ref->count);
 225         }
 226
 227         rcu_read_unlock_sched();
 228
 229         return ret;
 230 }
 231
 232 /**
 233  * percpu_ref_tryget_live - try to increment a live percpu refcount
 234  * @ref: percpu_ref to try-get
 235  *
 236  * Increment a percpu refcount unless it has already been killed.  Returns
 237  * %true on success; %false on failure.
 238  *
 239  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
 240  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
 241  * should be used.  After the confirm_kill callback is invoked, it's
 242  * guaranteed that no new reference will be given out by
 243  * percpu_ref_tryget_live().
 244  *
 245  * This function is safe to call as long as @ref is between init and exit.
 246  */
 247 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 248 {
 249         unsigned long __percpu *percpu_count;
 250         bool ret = false;
 251
 252         rcu_read_lock_sched();
 253
 254         if (__ref_is_percpu(ref, &percpu_count)) {
 255                 this_cpu_inc(*percpu_count);
 256                 ret = true;
 257         } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
 258                 ret = atomic_long_inc_not_zero(&ref->count);
 259         }
 260
 261         rcu_read_unlock_sched();
 262
 263         return ret;
 264 }
 265
 266 /**
 267  * percpu_ref_put_many - decrement a percpu refcount
 268  * @ref: percpu_ref to put
 269  * @nr: number of references to put
 270  *
 271  * Decrement the refcount, and if 0, call the release function (which was passed
 272  * to percpu_ref_init())
 273  *
 274  * This function is safe to call as long as @ref is between init and exit.
 275  */
 276 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
 277 {
 278         unsigned long __percpu *percpu_count;
 279
 280         rcu_read_lock_sched();
 281
 282         if (__ref_is_percpu(ref, &percpu_count))
 283                 this_cpu_sub(*percpu_count, nr);
 284         else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
 285                 ref->release(ref);
 286
 287         rcu_read_unlock_sched();
 288 }
 289
 290 /**
 291  * percpu_ref_put - decrement a percpu refcount
 292  * @ref: percpu_ref to put
 293  *
 294  * Decrement the refcount, and if 0, call the release function (which was passed
 295  * to percpu_ref_init())
 296  *
 297  * This function is safe to call as long as @ref is between init and exit.
 298  */
 299 static inline void percpu_ref_put(struct percpu_ref *ref)
 300 {
 301         percpu_ref_put_many(ref, 1);
 302 }
 303
 304 /**
 305  * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
 306  * @ref: percpu_ref to test
 307  *
 308  * Returns %true if @ref is dying or dead.
 309  *
 310  * This function is safe to call as long as @ref is between init and exit
 311  * and the caller is responsible for synchronizing against state changes.
 312  */
 313 static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
 314 {
 315         return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
 316 }
 317
 318 /**
 319  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 320  * @ref: percpu_ref to test
 321  *
 322  * Returns %true if @ref reached zero.
 323  *
 324  * This function is safe to call as long as @ref is between init and exit.
 325  */
 326 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 327 {
 328         unsigned long __percpu *percpu_count;
 329
 330         if (__ref_is_percpu(ref, &percpu_count))
 331                 return false;
 332         return !atomic_long_read(&ref->count);
 333 }
 334
 335 #endif