LXR linux/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/**
  35 * percpu_ref_init - initialize a percpu refcount
  36 * @ref: percpu_ref to initialize
  37 * @release: function which will be called when refcount hits 0
  38 *
  39 * Initializes the refcount in single atomic counter mode with a refcount of 1;
  40 * analagous to atomic_set(ref, 1).
  41 *
  42 * Note that @release must not sleep - it may potentially be called from RCU
  43 * callback context by percpu_ref_kill().
  44 */
  45int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
  46{
  47        atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
  48
  49        ref->pcpu_count = alloc_percpu(unsigned);
  50        if (!ref->pcpu_count)
  51                return -ENOMEM;
  52
  53        ref->release = release;
  54        return 0;
  55}
  56EXPORT_SYMBOL_GPL(percpu_ref_init);
  57
  58/**
  59 * percpu_ref_cancel_init - cancel percpu_ref_init()
  60 * @ref: percpu_ref to cancel init for
  61 *
  62 * Once a percpu_ref is initialized, its destruction is initiated by
  63 * percpu_ref_kill() and completes asynchronously, which can be painful to
  64 * do when destroying a half-constructed object in init failure path.
  65 *
  66 * This function destroys @ref without invoking @ref->release and the
  67 * memory area containing it can be freed immediately on return.  To
  68 * prevent accidental misuse, it's required that @ref has finished
  69 * percpu_ref_init(), whether successful or not, but never used.
  70 *
  71 * The weird name and usage restriction are to prevent people from using
  72 * this function by mistake for normal shutdown instead of
  73 * percpu_ref_kill().
  74 */
  75void percpu_ref_cancel_init(struct percpu_ref *ref)
  76{
  77        unsigned __percpu *pcpu_count = ref->pcpu_count;
  78        int cpu;
  79
  80        WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
  81
  82        if (pcpu_count) {
  83                for_each_possible_cpu(cpu)
  84                        WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
  85                free_percpu(ref->pcpu_count);
  86        }
  87}
  88EXPORT_SYMBOL_GPL(percpu_ref_cancel_init);
  89
  90static void percpu_ref_kill_rcu(struct rcu_head *rcu)
  91{
  92        struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
  93        unsigned __percpu *pcpu_count = ref->pcpu_count;
  94        unsigned count = 0;
  95        int cpu;
  96
  97        /* Mask out PCPU_REF_DEAD */
  98        pcpu_count = (unsigned __percpu *)
  99                (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
 100
 101        for_each_possible_cpu(cpu)
 102                count += *per_cpu_ptr(pcpu_count, cpu);
 103
 104        free_percpu(pcpu_count);
 105
 106        pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
 107
 108        /*
 109         * It's crucial that we sum the percpu counters _before_ adding the sum
 110         * to &ref->count; since gets could be happening on one cpu while puts
 111         * happen on another, adding a single cpu's count could cause
 112         * @ref->count to hit 0 before we've got a consistent value - but the
 113         * sum of all the counts will be consistent and correct.
 114         *
 115         * Subtracting the bias value then has to happen _after_ adding count to
 116         * &ref->count; we need the bias value to prevent &ref->count from
 117         * reaching 0 before we add the percpu counts. But doing it at the same
 118         * time is equivalent and saves us atomic operations:
 119         */
 120
 121        atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
 122
 123        /* @ref is viewed as dead on all CPUs, send out kill confirmation */
 124        if (ref->confirm_kill)
 125                ref->confirm_kill(ref);
 126
 127        /*
 128         * Now we're in single atomic_t mode with a consistent refcount, so it's
 129         * safe to drop our initial ref:
 130         */
 131        percpu_ref_put(ref);
 132}
 133
 134/**
 135 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
 136 * @ref: percpu_ref to kill
 137 * @confirm_kill: optional confirmation callback
 138 *
 139 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
 140 * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be
 141 * called after @ref is seen as dead from all CPUs - all further
 142 * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget()
 143 * for more details.
 144 *
 145 * Due to the way percpu_ref is implemented, @confirm_kill will be called
 146 * after at least one full RCU grace period has passed but this is an
 147 * implementation detail and callers must not depend on it.
 148 */
 149void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
 150                                 percpu_ref_func_t *confirm_kill)
 151{
 152        WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
 153                  "percpu_ref_kill() called more than once!\n");
 154
 155        ref->pcpu_count = (unsigned __percpu *)
 156                (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
 157        ref->confirm_kill = confirm_kill;
 158
 159        call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
 160}
 161EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
 162