Branch data Line data Source code
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 : 0 : int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
46 : : {
47 : 0 : atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
48 : :
49 : 0 : ref->pcpu_count = alloc_percpu(unsigned);
50 [ # # ]: 0 : if (!ref->pcpu_count)
51 : : return -ENOMEM;
52 : :
53 : 0 : ref->release = release;
54 : 0 : return 0;
55 : : }
56 : : EXPORT_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 : : */
75 : 0 : void percpu_ref_cancel_init(struct percpu_ref *ref)
76 : : {
77 : 0 : unsigned __percpu *pcpu_count = ref->pcpu_count;
78 : : int cpu;
79 : :
80 [ # # ][ # # ]: 0 : WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS);
[ # # ]
81 : :
82 [ # # ]: 0 : if (pcpu_count) {
83 [ # # ]: 0 : for_each_possible_cpu(cpu)
84 [ # # ][ # # ]: 0 : WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));
[ # # ]
85 : 0 : free_percpu(ref->pcpu_count);
86 : : }
87 : 0 : }
88 : : EXPORT_SYMBOL_GPL(percpu_ref_cancel_init);
89 : :
90 : 0 : static void percpu_ref_kill_rcu(struct rcu_head *rcu)
91 : : {
92 : 0 : struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
93 : 0 : unsigned __percpu *pcpu_count = ref->pcpu_count;
94 : : unsigned count = 0;
95 : : int cpu;
96 : :
97 : : /* Mask out PCPU_REF_DEAD */
98 : 0 : pcpu_count = (unsigned __percpu *)
99 : 0 : (((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK);
100 : :
101 [ # # ]: 0 : for_each_possible_cpu(cpu)
102 : 0 : count += *per_cpu_ptr(pcpu_count, cpu);
103 : :
104 : 0 : free_percpu(pcpu_count);
105 : :
106 : 0 : 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 : 0 : atomic_add((int) count - PCPU_COUNT_BIAS, &ref->count);
122 : :
123 : : /* @ref is viewed as dead on all CPUs, send out kill confirmation */
124 [ # # ]: 0 : if (ref->confirm_kill)
125 : 0 : 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 : 0 : }
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 : : */
149 : 0 : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
150 : : percpu_ref_func_t *confirm_kill)
151 : : {
152 [ # # ][ # # ]: 0 : WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,
[ # # ]
153 : : "percpu_ref_kill() called more than once!\n");
154 : :
155 : 0 : ref->pcpu_count = (unsigned __percpu *)
156 : 0 : (((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);
157 : 0 : ref->confirm_kill = confirm_kill;
158 : :
159 : 0 : call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);
160 : 0 : }
161 : : EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
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