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  */
  45 int 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  */
  74 void 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
  88 static 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  */
 147 void 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 }