3 * (C) 2012 Google, Inc.
4 * Author: Kent Overstreet <koverstreet@google.com>
6 * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
7 * atomic_dec_and_test() - but percpu.
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
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.
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.
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.
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().
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.
45 #ifndef _LINUX_PERCPU_REFCOUNT_H
46 #define _LINUX_PERCPU_REFCOUNT_H
48 #include <linux/atomic.h>
49 #include <linux/kernel.h>
50 #include <linux/percpu.h>
51 #include <linux/rcupdate.h>
54 typedef void (percpu_ref_func_t
)(struct percpu_ref
*);
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())
64 unsigned __percpu
*pcpu_count
;
65 percpu_ref_func_t
*release
;
69 int __must_check
percpu_ref_init(struct percpu_ref
*ref
,
70 percpu_ref_func_t
*release
);
71 void percpu_ref_kill(struct percpu_ref
*ref
);
73 #define PCPU_STATUS_BITS 2
74 #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
75 #define PCPU_REF_PTR 0
76 #define PCPU_REF_DEAD 1
78 #define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
81 * percpu_ref_get - increment a percpu refcount
82 * @ref: percpu_ref to get
84 * Analagous to atomic_inc().
86 static inline void percpu_ref_get(struct percpu_ref
*ref
)
88 unsigned __percpu
*pcpu_count
;
92 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
94 if (likely(REF_STATUS(pcpu_count
) == PCPU_REF_PTR
))
95 __this_cpu_inc(*pcpu_count
);
97 atomic_inc(&ref
->count
);
103 * percpu_ref_put - decrement a percpu refcount
104 * @ref: percpu_ref to put
106 * Decrement the refcount, and if 0, call the release function (which was passed
107 * to percpu_ref_init())
109 static inline void percpu_ref_put(struct percpu_ref
*ref
)
111 unsigned __percpu
*pcpu_count
;
115 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
117 if (likely(REF_STATUS(pcpu_count
) == PCPU_REF_PTR
))
118 __this_cpu_dec(*pcpu_count
);
119 else if (unlikely(atomic_dec_and_test(&ref
->count
)))
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