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-rw-r--r--lib/percpu-refcount.c128
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diff --git a/lib/percpu-refcount.c b/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: 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_release *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
57static 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;
61 unsigned count = 0;
62 int cpu;
63
64 pcpu_count = ACCESS_ONCE(ref->pcpu_count);
65
66 /* Mask out PCPU_REF_DEAD */
67 pcpu_count = (unsigned __percpu *)
68 (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
69
70 for_each_possible_cpu(cpu)
71 count += *per_cpu_ptr(pcpu_count, cpu);
72
73 free_percpu(pcpu_count);
74
75 pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
76
77 /*
78 * It's crucial that we sum the percpu counters _before_ adding the sum
79 * to &ref->count; since gets could be happening on one cpu while puts
80 * happen on another, adding a single cpu's count could cause
81 * @ref->count to hit 0 before we've got a consistent value - but the
82 * sum of all the counts will be consistent and correct.
83 *
84 * Subtracting the bias value then has to happen _after_ adding count to
85 * &ref->count; we need the bias value to prevent &ref->count from
86 * reaching 0 before we add the percpu counts. But doing it at the same
87 * time is equivalent and saves us atomic operations:
88 */
89
90 atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
91
92 /*
93 * Now we're in single atomic_t mode with a consistent refcount, so it's
94 * safe to drop our initial ref:
95 */
96 percpu_ref_put(ref);
97}
98
99/**
100 * percpu_ref_kill - safely drop initial ref
101 *
102 * Must be used to drop the initial ref on a percpu refcount; must be called
103 * precisely once before shutdown.
104 *
105 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
106 * percpu counters and dropping the initial ref.
107 */
108void percpu_ref_kill(struct percpu_ref *ref)
109{
110 unsigned __percpu *pcpu_count, *old, *new;
111
112 pcpu_count = ACCESS_ONCE(ref->pcpu_count);
113
114 do {
115 if (REF_STATUS(pcpu_count) == PCPU_REF_DEAD) {
116 WARN(1, "percpu_ref_kill() called more than once!\n");
117 return;
118 }
119
120 old = pcpu_count;
121 new = (unsigned __percpu *)
122 (((unsigned long) pcpu_count)|PCPU_REF_DEAD);
123
124 pcpu_count = cmpxchg(&ref->pcpu_count, old, new);
125 } while (pcpu_count != old);
126
127 call_rcu(&ref->rcu, percpu_ref_kill_rcu);
128}