lib/percpu-refcount.c

   1 #define pr_fmt(fmt) "%s: " fmt "\n", __func__
   2
   3 #include <linux/kernel.h>
   4 #include <linux/percpu-refcount.h>
   5
   6 /*
   7  * Initially, a percpu refcount is just a set of percpu counters. Initially, we
   8  * don't try to detect the ref hitting 0 - which means that get/put can just
   9  * increment or decrement the local counter. Note that the counter on a
  10  * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
  11  * percpu counters will all sum to the correct value
  12  *
  13  * (More precisely: because moduler arithmatic is commutative the sum of all the
  14  * pcpu_count vars will be equal to what it would have been if all the gets and
  15  * puts were done to a single integer, even if some of the percpu integers
  16  * overflow or underflow).
  17  *
  18  * The real trick to implementing percpu refcounts is shutdown. We can't detect
  19  * the ref hitting 0 on every put - this would require global synchronization
  20  * and defeat the whole purpose of using percpu refs.
  21  *
  22  * What we do is require the user to keep track of the initial refcount; we know
  23  * the ref can't hit 0 before the user drops the initial ref, so as long as we
  24  * convert to non percpu mode before the initial ref is dropped everything
  25  * works.
  26  *
  27  * Converting to non percpu mode is done with some RCUish stuff in
  28  * percpu_ref_kill. Additionally, we need a bias value so that the atomic_t
  29  * can't hit 0 before we've added up all the percpu refs.
  30  */
  31
  32 #define PCPU_COUNT_BIAS         (1U << 31)
  33
  34 static unsigned __percpu *pcpu_count_ptr(struct percpu_ref *ref)
  35 {
  36         return (unsigned __percpu *)(ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
  37 }
  38
  39 /**
  40  * percpu_ref_init - initialize a percpu refcount
  41  * @ref: percpu_ref to initialize
  42  * @release: function which will be called when refcount hits 0
  43  *
  44  * Initializes the refcount in single atomic counter mode with a refcount of 1;
  45  * analagous to atomic_set(ref, 1).
  46  *
  47  * Note that @release must not sleep - it may potentially be called from RCU
  48  * callback context by percpu_ref_kill().
  49  */
  50 int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
  51 {
  52         atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
  53
  54         ref->pcpu_count_ptr = (unsigned long)alloc_percpu(unsigned);
  55         if (!ref->pcpu_count_ptr)
  56                 return -ENOMEM;
  57
  58         ref->release = release;
  59         return 0;
  60 }
  61 EXPORT_SYMBOL_GPL(percpu_ref_init);
  62
  63 /**
  64  * percpu_ref_reinit - re-initialize a percpu refcount
  65  * @ref: perpcu_ref to re-initialize
  66  *
  67  * Re-initialize @ref so that it's in the same state as when it finished
  68  * percpu_ref_init().  @ref must have been initialized successfully, killed
  69  * and reached 0 but not exited.
  70  *
  71  * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
  72  * this function is in progress.
  73  */
  74 void percpu_ref_reinit(struct percpu_ref *ref)
  75 {
  76         unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
  77         int cpu;
  78
  79         BUG_ON(!pcpu_count);
  80         WARN_ON(!percpu_ref_is_zero(ref));
  81
  82         atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
  83
  84         /*
  85          * Restore per-cpu operation.  smp_store_release() is paired with
  86          * smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees
  87          * that the zeroing is visible to all percpu accesses which can see
  88          * the following PCPU_REF_DEAD clearing.
  89          */
  90         for_each_possible_cpu(cpu)
  91                 *per_cpu_ptr(pcpu_count, cpu) = 0;
  92
  93         smp_store_release(&ref->pcpu_count_ptr,
  94                           ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
  95 }
  96 EXPORT_SYMBOL_GPL(percpu_ref_reinit);
  97
  98 /**
  99  * percpu_ref_exit - undo percpu_ref_init()
 100  * @ref: percpu_ref to exit
 101  *
 102  * This function exits @ref.  The caller is responsible for ensuring that
 103  * @ref is no longer in active use.  The usual places to invoke this
 104  * function from are the @ref->release() callback or in init failure path
 105  * where percpu_ref_init() succeeded but other parts of the initialization
 106  * of the embedding object failed.
 107  */
 108 void percpu_ref_exit(struct percpu_ref *ref)
 109 {
 110         unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
 111
 112         if (pcpu_count) {
 113                 free_percpu(pcpu_count);
 114                 ref->pcpu_count_ptr = PCPU_REF_DEAD;
 115         }
 116 }
 117 EXPORT_SYMBOL_GPL(percpu_ref_exit);
 118
 119 static void percpu_ref_kill_rcu(struct rcu_head *rcu)
 120 {
 121         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
 122         unsigned __percpu *pcpu_count = pcpu_count_ptr(ref);
 123         unsigned count = 0;
 124         int cpu;
 125
 126         for_each_possible_cpu(cpu)
 127                 count += *per_cpu_ptr(pcpu_count, cpu);
 128
 129         pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
 130
 131         /*
 132          * It's crucial that we sum the percpu counters _before_ adding the sum
 133          * to &ref->count; since gets could be happening on one cpu while puts
 134          * happen on another, adding a single cpu's count could cause
 135          * @ref->count to hit 0 before we've got a consistent value - but the
 136          * sum of all the counts will be consistent and correct.
 137          *
 138          * Subtracting the bias value then has to happen _after_ adding count to
 139          * &ref->count; we need the bias value to prevent &ref->count from
 140          * reaching 0 before we add the percpu counts. But doing it at the same
 141          * time is equivalent and saves us atomic operations:
 142          */
 143
 144         atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
 145
 146         WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)",
 147                   atomic_read(&ref->count));
 148
 149         /* @ref is viewed as dead on all CPUs, send out kill confirmation */
 150         if (ref->confirm_kill)
 151                 ref->confirm_kill(ref);
 152
 153         /*
 154          * Now we're in single atomic_t mode with a consistent refcount, so it's
 155          * safe to drop our initial ref:
 156          */
 157         percpu_ref_put(ref);
 158 }
 159
 160 /**
 161  * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
 162  * @ref: percpu_ref to kill
 163  * @confirm_kill: optional confirmation callback
 164  *
 165  * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
 166  * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
 167  * called after @ref is seen as dead from all CPUs - all further
 168  * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
 169  * for more details.
 170  *
 171  * Due to the way percpu_ref is implemented, @confirm_kill will be called
 172  * after at least one full RCU grace period has passed but this is an
 173  * implementation detail and callers must not depend on it.
 174  */
 175 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 176                                  percpu_ref_func_t *confirm_kill)
 177 {
 178         WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,
 179                   "percpu_ref_kill() called more than once!\n");
 180
 181         ref->pcpu_count_ptr |= PCPU_REF_DEAD;
 182         ref->confirm_kill = confirm_kill;
 183
 184         call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
 185 }
 186 EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);