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1 | /* |
2 | * Percpu refcounts: | |
3 | * (C) 2012 Google, Inc. | |
4 | * Author: Kent Overstreet <koverstreet@google.com> | |
5 | * | |
6 | * This implements a refcount with similar semantics to atomic_t - atomic_inc(), | |
7 | * atomic_dec_and_test() - but percpu. | |
8 | * | |
9 | * There's one important difference between percpu refs and normal atomic_t | |
10 | * refcounts; you have to keep track of your initial refcount, and then when you | |
11 | * start shutting down you call percpu_ref_kill() _before_ dropping the initial | |
12 | * refcount. | |
13 | * | |
14 | * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less | |
15 | * than an atomic_t - this is because of the way shutdown works, see | |
16 | * percpu_ref_kill()/PCPU_COUNT_BIAS. | |
17 | * | |
18 | * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the | |
19 | * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() | |
20 | * puts the ref back in single atomic_t mode, collecting the per cpu refs and | |
21 | * issuing the appropriate barriers, and then marks the ref as shutting down so | |
22 | * that percpu_ref_put() will check for the ref hitting 0. After it returns, | |
23 | * it's safe to drop the initial ref. | |
24 | * | |
25 | * USAGE: | |
26 | * | |
27 | * See fs/aio.c for some example usage; it's used there for struct kioctx, which | |
28 | * is created when userspaces calls io_setup(), and destroyed when userspace | |
29 | * calls io_destroy() or the process exits. | |
30 | * | |
31 | * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it | |
32 | * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove | |
33 | * the kioctx from the proccess's list of kioctxs - after that, there can't be | |
34 | * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop | |
35 | * the initial ref with percpu_ref_put(). | |
36 | * | |
37 | * Code that does a two stage shutdown like this often needs some kind of | |
38 | * explicit synchronization to ensure the initial refcount can only be dropped | |
39 | * once - percpu_ref_kill() does this for you, it returns true once and false if | |
40 | * someone else already called it. The aio code uses it this way, but it's not | |
41 | * necessary if the code has some other mechanism to synchronize teardown. | |
42 | * around. | |
43 | */ | |
44 | ||
45 | #ifndef _LINUX_PERCPU_REFCOUNT_H | |
46 | #define _LINUX_PERCPU_REFCOUNT_H | |
47 | ||
48 | #include <linux/atomic.h> | |
49 | #include <linux/kernel.h> | |
50 | #include <linux/percpu.h> | |
51 | #include <linux/rcupdate.h> | |
52 | ||
53 | struct percpu_ref; | |
54 | typedef void (percpu_ref_release)(struct percpu_ref *); | |
55 | ||
56 | struct percpu_ref { | |
57 | atomic_t count; | |
58 | /* | |
59 | * The low bit of the pointer indicates whether the ref is in percpu | |
60 | * mode; if set, then get/put will manipulate the atomic_t (this is a | |
61 | * hack because we need to keep the pointer around for | |
62 | * percpu_ref_kill_rcu()) | |
63 | */ | |
64 | unsigned __percpu *pcpu_count; | |
65 | percpu_ref_release *release; | |
66 | struct rcu_head rcu; | |
67 | }; | |
68 | ||
69 | int percpu_ref_init(struct percpu_ref *, percpu_ref_release *); | |
70 | void percpu_ref_kill(struct percpu_ref *ref); | |
71 | ||
72 | #define PCPU_STATUS_BITS 2 | |
73 | #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1) | |
74 | #define PCPU_REF_PTR 0 | |
75 | #define PCPU_REF_DEAD 1 | |
76 | ||
77 | #define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK) | |
78 | ||
79 | /** | |
80 | * percpu_ref_get - increment a percpu refcount | |
81 | * | |
82 | * Analagous to atomic_inc(). | |
83 | */ | |
84 | static inline void percpu_ref_get(struct percpu_ref *ref) | |
85 | { | |
86 | unsigned __percpu *pcpu_count; | |
87 | ||
88 | preempt_disable(); | |
89 | ||
90 | pcpu_count = ACCESS_ONCE(ref->pcpu_count); | |
91 | ||
92 | if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) | |
93 | __this_cpu_inc(*pcpu_count); | |
94 | else | |
95 | atomic_inc(&ref->count); | |
96 | ||
97 | preempt_enable(); | |
98 | } | |
99 | ||
100 | /** | |
101 | * percpu_ref_put - decrement a percpu refcount | |
102 | * | |
103 | * Decrement the refcount, and if 0, call the release function (which was passed | |
104 | * to percpu_ref_init()) | |
105 | */ | |
106 | static inline void percpu_ref_put(struct percpu_ref *ref) | |
107 | { | |
108 | unsigned __percpu *pcpu_count; | |
109 | ||
110 | preempt_disable(); | |
111 | ||
112 | pcpu_count = ACCESS_ONCE(ref->pcpu_count); | |
113 | ||
114 | if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) | |
115 | __this_cpu_dec(*pcpu_count); | |
116 | else if (unlikely(atomic_dec_and_test(&ref->count))) | |
117 | ref->release(ref); | |
118 | ||
119 | preempt_enable(); | |
120 | } | |
121 | ||
122 | #endif |