Root/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 */
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}
56EXPORT_SYMBOL_GPL(percpu_ref_init);
57
58/**
59 * percpu_ref_cancel_init - cancel percpu_ref_init()
60 * @ref: percpu_ref to cancel init for
61 *
62 * Once a percpu_ref is initialized, its destruction is initiated by
63 * percpu_ref_kill() and completes asynchronously, which can be painful to
64 * do when destroying a half-constructed object in init failure path.
65 *
66 * This function destroys @ref without invoking @ref->release and the
67 * memory area containing it can be freed immediately on return. To
68 * prevent accidental misuse, it's required that @ref has finished
69 * percpu_ref_init(), whether successful or not, but never used.
70 *
71 * The weird name and usage restriction are to prevent people from using
72 * this function by mistake for normal shutdown instead of
73 * percpu_ref_kill().
74 */
75void percpu_ref_cancel_init(struct percpu_ref *ref)
76{
77    unsigned __percpu *pcpu_count = ref->pcpu_count;
78    int cpu;
79
80    WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
81
82    if (pcpu_count) {
83        for_each_possible_cpu(cpu)
84            WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
85        free_percpu(ref->pcpu_count);
86    }
87}
88EXPORT_SYMBOL_GPL(percpu_ref_cancel_init);
89
90static void percpu_ref_kill_rcu(struct rcu_head *rcu)
91{
92    struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
93    unsigned __percpu *pcpu_count = ref->pcpu_count;
94    unsigned count = 0;
95    int cpu;
96
97    /* Mask out PCPU_REF_DEAD */
98    pcpu_count = (unsigned __percpu *)
99        (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
100
101    for_each_possible_cpu(cpu)
102        count += *per_cpu_ptr(pcpu_count, cpu);
103
104    free_percpu(pcpu_count);
105
106    pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) count);
107
108    /*
109     * It's crucial that we sum the percpu counters _before_ adding the sum
110     * to &ref->count; since gets could be happening on one cpu while puts
111     * happen on another, adding a single cpu's count could cause
112     * @ref->count to hit 0 before we've got a consistent value - but the
113     * sum of all the counts will be consistent and correct.
114     *
115     * Subtracting the bias value then has to happen _after_ adding count to
116     * &ref->count; we need the bias value to prevent &ref->count from
117     * reaching 0 before we add the percpu counts. But doing it at the same
118     * time is equivalent and saves us atomic operations:
119     */
120
121    atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
122
123    /* @ref is viewed as dead on all CPUs, send out kill confirmation */
124    if (ref->confirm_kill)
125        ref->confirm_kill(ref);
126
127    /*
128     * Now we're in single atomic_t mode with a consistent refcount, so it's
129     * safe to drop our initial ref:
130     */
131    percpu_ref_put(ref);
132}
133
134/**
135 * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
136 * @ref: percpu_ref to kill
137 * @confirm_kill: optional confirmation callback
138 *
139 * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
140 * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
141 * called after @ref is seen as dead from all CPUs - all further
142 * invocations of percpu_ref_tryget() will fail. See percpu_ref_tryget()
143 * for more details.
144 *
145 * Due to the way percpu_ref is implemented, @confirm_kill will be called
146 * after at least one full RCU grace period has passed but this is an
147 * implementation detail and callers must not depend on it.
148 */
149void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
150                 percpu_ref_func_t *confirm_kill)
151{
152    WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
153          "percpu_ref_kill() called more than once!\n");
154
155    ref->pcpu_count = (unsigned __percpu *)
156        (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
157    ref->confirm_kill = confirm_kill;
158
159    call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
160}
161EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
162

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