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