lib/percpu-refcount.c

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