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 static void percpu_ref_kill_rcu(struct rcu_head *rcu)
  58 {
  59         struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
  60         unsigned __percpu *pcpu_count = ref->pcpu_count;
  61         unsigned count = 0;
  62         int cpu;
  63
  64         /* Mask out PCPU_REF_DEAD */
  65         pcpu_count = (unsigned __percpu *)
  66                 (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
  67
  68         for_each_possible_cpu(cpu)
  69                 count += *per_cpu_ptr(pcpu_count, cpu);
  70
  71         free_percpu(pcpu_count);
  72
  73         pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
  74
  75         /*
  76          * It's crucial that we sum the percpu counters _before_ adding the sum
  77          * to &ref->count; since gets could be happening on one cpu while puts
  78          * happen on another, adding a single cpu's count could cause
  79          * @ref->count to hit 0 before we've got a consistent value - but the
  80          * sum of all the counts will be consistent and correct.
  81          *
  82          * Subtracting the bias value then has to happen _after_ adding count to
  83          * &ref->count; we need the bias value to prevent &ref->count from
  84          * reaching 0 before we add the percpu counts. But doing it at the same
  85          * time is equivalent and saves us atomic operations:
  86          */
  87
  88         atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
  89
  90         /*
  91          * Now we're in single atomic_t mode with a consistent refcount, so it's
  92          * safe to drop our initial ref:
  93          */
  94         percpu_ref_put(ref);
  95 }
  96
  97 /**
  98  * percpu_ref_kill - safely drop initial ref
  99  * @ref: percpu_ref to kill
 100  *
 101  * Must be used to drop the initial ref on a percpu refcount; must be called
 102  * precisely once before shutdown.
 103  *
 104  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
 105  * percpu counters and dropping the initial ref.
 106  */
 107 void percpu_ref_kill(struct percpu_ref *ref)
 108 {
 109         WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
 110                   "percpu_ref_kill() called more than once!\n");
 111
 112         ref->pcpu_count = (unsigned __percpu *)
 113                 (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
 114
 115         call_rcu(&ref->rcu, percpu_ref_kill_rcu);
 116 }