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  * calls percpu_ref_kill(), then hlist_del_rcu() and synchronize_rcu() to remove
  34  * the kioctx from the proccess's list of kioctxs - after that, there can't be
  35  * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop
  36  * the initial ref with percpu_ref_put().
  37  *
  38  * Code that does a two stage shutdown like this often needs some kind of
  39  * explicit synchronization to ensure the initial refcount can only be dropped
  40  * once - percpu_ref_kill() does this for you, it returns true once and false if
  41  * someone else already called it. The aio code uses it this way, but it's not
  42  * necessary if the code has some other mechanism to synchronize teardown.
  43  * around.
  44  */
  45
  46 #ifndef _LINUX_PERCPU_REFCOUNT_H
  47 #define _LINUX_PERCPU_REFCOUNT_H
  48
  49 #include <linux/atomic.h>
  50 #include <linux/kernel.h>
  51 #include <linux/percpu.h>
  52 #include <linux/rcupdate.h>
  53 #include <linux/gfp.h>
  54
  55 struct percpu_ref;
  56 typedef void (percpu_ref_func_t)(struct percpu_ref *);
  57
  58 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
  59 enum {
  60         __PERCPU_REF_ATOMIC     = 1LU << 0,     /* operating in atomic mode */
  61         __PERCPU_REF_DEAD       = 1LU << 1,     /* (being) killed */
  62         __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
  63
  64         __PERCPU_REF_FLAG_BITS  = 2,
  65 };
  66
  67 /* @flags for percpu_ref_init() */
  68 enum {
  69         /*
  70          * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
  71          * operation using percpu_ref_switch_to_percpu().  If initialized
  72          * with this flag, the ref will stay in atomic mode until
  73          * percpu_ref_switch_to_percpu() is invoked on it.
  74          */
  75         PERCPU_REF_INIT_ATOMIC  = 1 << 0,
  76
  77         /*
  78          * Start dead w/ ref == 0 in atomic mode.  Must be revived with
  79          * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
  80          */
  81         PERCPU_REF_INIT_DEAD    = 1 << 1,
  82 };
  83
  84 struct percpu_ref {
  85         atomic_long_t           count;
  86         /*
  87          * The low bit of the pointer indicates whether the ref is in percpu
  88          * mode; if set, then get/put will manipulate the atomic_t.
  89          */
  90         unsigned long           percpu_count_ptr;
  91         percpu_ref_func_t       *release;
  92         percpu_ref_func_t       *confirm_switch;
  93         bool                    force_atomic:1;
  94         struct rcu_head         rcu;
  95 };
  96
  97 int __must_check percpu_ref_init(struct percpu_ref *ref,
  98                                  percpu_ref_func_t *release, unsigned int flags,
  99                                  gfp_t gfp);
 100 void percpu_ref_exit(struct percpu_ref *ref);
 101 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 102                                  percpu_ref_func_t *confirm_switch);
 103 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
 104 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
 105 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 106                                  percpu_ref_func_t *confirm_kill);
 107 void percpu_ref_reinit(struct percpu_ref *ref);
 108
 109 /**
 110  * percpu_ref_kill - drop the initial ref
 111  * @ref: percpu_ref to kill
 112  *
 113  * Must be used to drop the initial ref on a percpu refcount; must be called
 114  * precisely once before shutdown.
 115  *
 116  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
 117  * percpu counters and dropping the initial ref.
 118  */
 119 static inline void percpu_ref_kill(struct percpu_ref *ref)
 120 {
 121         percpu_ref_kill_and_confirm(ref, NULL);
 122 }
 123
 124 /*
 125  * Internal helper.  Don't use outside percpu-refcount proper.  The
 126  * function doesn't return the pointer and let the caller test it for NULL
 127  * because doing so forces the compiler to generate two conditional
 128  * branches as it can't assume that @ref->percpu_count is not NULL.
 129  */
 130 static inline bool __ref_is_percpu(struct percpu_ref *ref,
 131                                           unsigned long __percpu **percpu_countp)
 132 {
 133         unsigned long percpu_ptr;
 134
 135         /*
 136          * The value of @ref->percpu_count_ptr is tested for
 137          * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
 138          * used as a pointer.  If the compiler generates a separate fetch
 139          * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
 140          * between contaminating the pointer value, meaning that
 141          * READ_ONCE() is required when fetching it.
 142          *
 143          * The smp_read_barrier_depends() implied by READ_ONCE() pairs
 144          * with smp_store_release() in __percpu_ref_switch_to_percpu().
 145          */
 146         percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
 147
 148         /*
 149          * Theoretically, the following could test just ATOMIC; however,
 150          * then we'd have to mask off DEAD separately as DEAD may be
 151          * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
 152          * implies ATOMIC anyway.  Test them together.
 153          */
 154         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
 155                 return false;
 156
 157         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
 158         return true;
 159 }
 160
 161 /**
 162  * percpu_ref_get_many - increment a percpu refcount
 163  * @ref: percpu_ref to get
 164  * @nr: number of references to get
 165  *
 166  * Analogous to atomic_long_add().
 167  *
 168  * This function is safe to call as long as @ref is between init and exit.
 169  */
 170 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
 171 {
 172         unsigned long __percpu *percpu_count;
 173
 174         rcu_read_lock_sched();
 175
 176         if (__ref_is_percpu(ref, &percpu_count))
 177                 this_cpu_add(*percpu_count, nr);
 178         else
 179                 atomic_long_add(nr, &ref->count);
 180
 181         rcu_read_unlock_sched();
 182 }
 183
 184 /**
 185  * percpu_ref_get - increment a percpu refcount
 186  * @ref: percpu_ref to get
 187  *
 188  * Analagous to atomic_long_inc().
 189  *
 190  * This function is safe to call as long as @ref is between init and exit.
 191  */
 192 static inline void percpu_ref_get(struct percpu_ref *ref)
 193 {
 194         percpu_ref_get_many(ref, 1);
 195 }
 196
 197 /**
 198  * percpu_ref_tryget - try to increment a percpu refcount
 199  * @ref: percpu_ref to try-get
 200  *
 201  * Increment a percpu refcount unless its count already reached zero.
 202  * Returns %true on success; %false on failure.
 203  *
 204  * This function is safe to call as long as @ref is between init and exit.
 205  */
 206 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 207 {
 208         unsigned long __percpu *percpu_count;
 209         bool ret;
 210
 211         rcu_read_lock_sched();
 212
 213         if (__ref_is_percpu(ref, &percpu_count)) {
 214                 this_cpu_inc(*percpu_count);
 215                 ret = true;
 216         } else {
 217                 ret = atomic_long_inc_not_zero(&ref->count);
 218         }
 219
 220         rcu_read_unlock_sched();
 221
 222         return ret;
 223 }
 224
 225 /**
 226  * percpu_ref_tryget_live - try to increment a live percpu refcount
 227  * @ref: percpu_ref to try-get
 228  *
 229  * Increment a percpu refcount unless it has already been killed.  Returns
 230  * %true on success; %false on failure.
 231  *
 232  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
 233  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
 234  * should be used.  After the confirm_kill callback is invoked, it's
 235  * guaranteed that no new reference will be given out by
 236  * percpu_ref_tryget_live().
 237  *
 238  * This function is safe to call as long as @ref is between init and exit.
 239  */
 240 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
 241 {
 242         unsigned long __percpu *percpu_count;
 243         bool ret = false;
 244
 245         rcu_read_lock_sched();
 246
 247         if (__ref_is_percpu(ref, &percpu_count)) {
 248                 this_cpu_inc(*percpu_count);
 249                 ret = true;
 250         } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
 251                 ret = atomic_long_inc_not_zero(&ref->count);
 252         }
 253
 254         rcu_read_unlock_sched();
 255
 256         return ret;
 257 }
 258
 259 /**
 260  * percpu_ref_put_many - decrement a percpu refcount
 261  * @ref: percpu_ref to put
 262  * @nr: number of references to put
 263  *
 264  * Decrement the refcount, and if 0, call the release function (which was passed
 265  * to percpu_ref_init())
 266  *
 267  * This function is safe to call as long as @ref is between init and exit.
 268  */
 269 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
 270 {
 271         unsigned long __percpu *percpu_count;
 272
 273         rcu_read_lock_sched();
 274
 275         if (__ref_is_percpu(ref, &percpu_count))
 276                 this_cpu_sub(*percpu_count, nr);
 277         else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
 278                 ref->release(ref);
 279
 280         rcu_read_unlock_sched();
 281 }
 282
 283 /**
 284  * percpu_ref_put - decrement a percpu refcount
 285  * @ref: percpu_ref to put
 286  *
 287  * Decrement the refcount, and if 0, call the release function (which was passed
 288  * to percpu_ref_init())
 289  *
 290  * This function is safe to call as long as @ref is between init and exit.
 291  */
 292 static inline void percpu_ref_put(struct percpu_ref *ref)
 293 {
 294         percpu_ref_put_many(ref, 1);
 295 }
 296
 297 /**
 298  * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
 299  * @ref: percpu_ref to test
 300  *
 301  * Returns %true if @ref is dying or dead.
 302  *
 303  * This function is safe to call as long as @ref is between init and exit
 304  * and the caller is responsible for synchronizing against state changes.
 305  */
 306 static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
 307 {
 308         return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
 309 }
 310
 311 /**
 312  * percpu_ref_is_zero - test whether a percpu refcount reached zero
 313  * @ref: percpu_ref to test
 314  *
 315  * Returns %true if @ref reached zero.
 316  *
 317  * This function is safe to call as long as @ref is between init and exit.
 318  */
 319 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
 320 {
 321         unsigned long __percpu *percpu_count;
 322
 323         if (__ref_is_percpu(ref, &percpu_count))
 324                 return false;
 325         return !atomic_long_read(&ref->count);
 326 }
 327
 328 #endif