lib/percpu-refcount.c

   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
  36 static DEFINE_SPINLOCK(percpu_ref_switch_lock);
  37 static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
  38
  39 static 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  */
  59 int 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 }
  89 EXPORT_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  */
 101 void 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 }
 112 EXPORT_SYMBOL_GPL(percpu_ref_exit);
 113
 114 static 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
 126 static 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
 161 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
 162 {
 163 }
 164
 165 static 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
 187 static 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
 212 static 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  */
 251 void 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  */
 282 void 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  */
 309 void 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 }
 325 EXPORT_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  */
 338 void 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 }
 352 EXPORT_SYMBOL_GPL(percpu_ref_reinit);