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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 | |
ac899061 TH |
36 | * @ref: percpu_ref to initialize |
37 | * @release: function which will be called when refcount hits 0 | |
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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 | */ | |
ac899061 | 45 | int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release) |
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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; | |
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 | |
ac899061 | 101 | * @ref: percpu_ref to kill |
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102 | * |
103 | * Must be used to drop the initial ref on a percpu refcount; must be called | |
104 | * precisely once before shutdown. | |
105 | * | |
106 | * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the | |
107 | * percpu counters and dropping the initial ref. | |
108 | */ | |
109 | void percpu_ref_kill(struct percpu_ref *ref) | |
110 | { | |
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111 | WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD, |
112 | "percpu_ref_kill() called more than once!\n"); | |
215e262f | 113 | |
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114 | ref->pcpu_count = (unsigned __percpu *) |
115 | (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD); | |
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116 | |
117 | call_rcu(&ref->rcu, percpu_ref_kill_rcu); | |
118 | } |