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 DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
  37
  38 static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
  39 {
  40         return (unsigned long __percpu *)
  41                 (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
  42 }
  43
  44 /**
  45  * percpu_ref_init - initialize a percpu refcount
  46  * @ref: percpu_ref to initialize
  47  * @release: function which will be called when refcount hits 0
  48  * @flags: PERCPU_REF_INIT_* flags
  49  * @gfp: allocation mask to use
  50  *
  51  * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a
  52  * refcount of 1; analagous to atomic_long_set(ref, 1).  See the
  53  * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
  54  *
  55  * Note that @release must not sleep - it may potentially be called from RCU
  56  * callback context by percpu_ref_kill().
  57  */
  58 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
  59                     unsigned int flags, gfp_t gfp)
  60 {
  61         size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
  62                              __alignof__(unsigned long));
  63         unsigned long start_count = 0;
  64
  65         ref->percpu_count_ptr = (unsigned long)
  66                 __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
  67         if (!ref->percpu_count_ptr)
  68                 return -ENOMEM;
  69
  70         ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
  71
  72         if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
  73                 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
  74         else
  75                 start_count += PERCPU_COUNT_BIAS;
  76
  77         if (flags & PERCPU_REF_INIT_DEAD)
  78                 ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
  79         else
  80                 start_count++;
  81
  82         atomic_long_set(&ref->count, start_count);
  83
  84         ref->release = release;
  85         return 0;
  86 }
  87 EXPORT_SYMBOL_GPL(percpu_ref_init);
  88
  89 /**
  90  * percpu_ref_exit - undo percpu_ref_init()
  91  * @ref: percpu_ref to exit
  92  *
  93  * This function exits @ref.  The caller is responsible for ensuring that
  94  * @ref is no longer in active use.  The usual places to invoke this
  95  * function from are the @ref->release() callback or in init failure path
  96  * where percpu_ref_init() succeeded but other parts of the initialization
  97  * of the embedding object failed.
  98  */
  99 void percpu_ref_exit(struct percpu_ref *ref)
 100 {
 101         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 102
 103         if (percpu_count) {
 104                 free_percpu(percpu_count);
 105                 ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
 106         }
 107 }
 108 EXPORT_SYMBOL_GPL(percpu_ref_exit);
 109
 110 static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
 111 {
 112         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
 113
 114         ref->confirm_switch(ref);
 115         ref->confirm_switch = NULL;
 116         wake_up_all(&percpu_ref_switch_waitq);
 117
 118         /* drop ref from percpu_ref_switch_to_atomic() */
 119         percpu_ref_put(ref);
 120 }
 121
 122 static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
 123 {
 124         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
 125         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 126         unsigned long count = 0;
 127         int cpu;
 128
 129         for_each_possible_cpu(cpu)
 130                 count += *per_cpu_ptr(percpu_count, cpu);
 131
 132         pr_debug("global %ld percpu %ld",
 133                  atomic_long_read(&ref->count), (long)count);
 134
 135         /*
 136          * It's crucial that we sum the percpu counters _before_ adding the sum
 137          * to &ref->count; since gets could be happening on one cpu while puts
 138          * happen on another, adding a single cpu's count could cause
 139          * @ref->count to hit 0 before we've got a consistent value - but the
 140          * sum of all the counts will be consistent and correct.
 141          *
 142          * Subtracting the bias value then has to happen _after_ adding count to
 143          * &ref->count; we need the bias value to prevent &ref->count from
 144          * reaching 0 before we add the percpu counts. But doing it at the same
 145          * time is equivalent and saves us atomic operations:
 146          */
 147         atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
 148
 149         WARN_ONCE(atomic_long_read(&ref->count) <= 0,
 150                   "percpu ref (%pf) <= 0 (%ld) after switching to atomic",
 151                   ref->release, atomic_long_read(&ref->count));
 152
 153         /* @ref is viewed as dead on all CPUs, send out switch confirmation */
 154         percpu_ref_call_confirm_rcu(rcu);
 155 }
 156
 157 static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
 158 {
 159 }
 160
 161 static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 162                                           percpu_ref_func_t *confirm_switch)
 163 {
 164         if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) {
 165                 /* switching from percpu to atomic */
 166                 ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
 167
 168                 /*
 169                  * Non-NULL ->confirm_switch is used to indicate that
 170                  * switching is in progress.  Use noop one if unspecified.
 171                  */
 172                 WARN_ON_ONCE(ref->confirm_switch);
 173                 ref->confirm_switch =
 174                         confirm_switch ?: percpu_ref_noop_confirm_switch;
 175
 176                 percpu_ref_get(ref);    /* put after confirmation */
 177                 call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
 178         } else if (confirm_switch) {
 179                 /*
 180                  * Somebody already set ATOMIC.  Switching may still be in
 181                  * progress.  @confirm_switch must be invoked after the
 182                  * switching is complete and a full sched RCU grace period
 183                  * has passed.  Wait synchronously for the previous
 184                  * switching and schedule @confirm_switch invocation.
 185                  */
 186                 wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
 187                 ref->confirm_switch = confirm_switch;
 188
 189                 percpu_ref_get(ref);    /* put after confirmation */
 190                 call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
 191         }
 192 }
 193
 194 /**
 195  * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
 196  * @ref: percpu_ref to switch to atomic mode
 197  * @confirm_switch: optional confirmation callback
 198  *
 199  * There's no reason to use this function for the usual reference counting.
 200  * Use percpu_ref_kill[_and_confirm]().
 201  *
 202  * Schedule switching of @ref to atomic mode.  All its percpu counts will
 203  * be collected to the main atomic counter.  On completion, when all CPUs
 204  * are guaraneed to be in atomic mode, @confirm_switch, which may not
 205  * block, is invoked.  This function may be invoked concurrently with all
 206  * the get/put operations and can safely be mixed with kill and reinit
 207  * operations.  Note that @ref will stay in atomic mode across kill/reinit
 208  * cycles until percpu_ref_switch_to_percpu() is called.
 209  *
 210  * This function normally doesn't block and can be called from any context
 211  * but it may block if @confirm_kill is specified and @ref is already in
 212  * the process of switching to atomic mode.  In such cases, @confirm_switch
 213  * will be invoked after the switching is complete.
 214  *
 215  * Due to the way percpu_ref is implemented, @confirm_switch will be called
 216  * after at least one full sched RCU grace period has passed but this is an
 217  * implementation detail and must not be depended upon.
 218  */
 219 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
 220                                  percpu_ref_func_t *confirm_switch)
 221 {
 222         ref->force_atomic = true;
 223         __percpu_ref_switch_to_atomic(ref, confirm_switch);
 224 }
 225
 226 static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 227 {
 228         unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
 229         int cpu;
 230
 231         BUG_ON(!percpu_count);
 232
 233         if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
 234                 return;
 235
 236         wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
 237
 238         atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
 239
 240         /*
 241          * Restore per-cpu operation.  smp_store_release() is paired with
 242          * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
 243          * that the zeroing is visible to all percpu accesses which can see
 244          * the following __PERCPU_REF_ATOMIC clearing.
 245          */
 246         for_each_possible_cpu(cpu)
 247                 *per_cpu_ptr(percpu_count, cpu) = 0;
 248
 249         smp_store_release(&ref->percpu_count_ptr,
 250                           ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
 251 }
 252
 253 /**
 254  * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
 255  * @ref: percpu_ref to switch to percpu mode
 256  *
 257  * There's no reason to use this function for the usual reference counting.
 258  * To re-use an expired ref, use percpu_ref_reinit().
 259  *
 260  * Switch @ref to percpu mode.  This function may be invoked concurrently
 261  * with all the get/put operations and can safely be mixed with kill and
 262  * reinit operations.  This function reverses the sticky atomic state set
 263  * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is
 264  * dying or dead, the actual switching takes place on the following
 265  * percpu_ref_reinit().
 266  *
 267  * This function normally doesn't block and can be called from any context
 268  * but it may block if @ref is in the process of switching to atomic mode
 269  * by percpu_ref_switch_atomic().
 270  */
 271 void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
 272 {
 273         ref->force_atomic = false;
 274
 275         /* a dying or dead ref can't be switched to percpu mode w/o reinit */
 276         if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
 277                 __percpu_ref_switch_to_percpu(ref);
 278 }
 279
 280 /**
 281  * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
 282  * @ref: percpu_ref to kill
 283  * @confirm_kill: optional confirmation callback
 284  *
 285  * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
 286  * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
 287  * called after @ref is seen as dead from all CPUs at which point all
 288  * further invocations of percpu_ref_tryget_live() will fail.  See
 289  * percpu_ref_tryget_live() for details.
 290  *
 291  * This function normally doesn't block and can be called from any context
 292  * but it may block if @confirm_kill is specified and @ref is in the
 293  * process of switching to atomic mode by percpu_ref_switch_atomic().
 294  *
 295  * Due to the way percpu_ref is implemented, @confirm_switch will be called
 296  * after at least one full sched RCU grace period has passed but this is an
 297  * implementation detail and must not be depended upon.
 298  */
 299 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 300                                  percpu_ref_func_t *confirm_kill)
 301 {
 302         WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
 303                   "%s called more than once on %pf!", __func__, ref->release);
 304
 305         ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
 306         __percpu_ref_switch_to_atomic(ref, confirm_kill);
 307         percpu_ref_put(ref);
 308 }
 309 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
 310
 311 /**
 312  * percpu_ref_reinit - re-initialize a percpu refcount
 313  * @ref: perpcu_ref to re-initialize
 314  *
 315  * Re-initialize @ref so that it's in the same state as when it finished
 316  * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been
 317  * initialized successfully and reached 0 but not exited.
 318  *
 319  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
 320  * this function is in progress.
 321  */
 322 void percpu_ref_reinit(struct percpu_ref *ref)
 323 {
 324         WARN_ON_ONCE(!percpu_ref_is_zero(ref));
 325
 326         ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
 327         percpu_ref_get(ref);
 328         if (!ref->force_atomic)
 329                 __percpu_ref_switch_to_percpu(ref);
 330 }
 331 EXPORT_SYMBOL_GPL(percpu_ref_reinit);