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