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
;
66 percpu_ref_func_t
*confirm_kill
;
70 int __must_check
percpu_ref_init(struct percpu_ref
*ref
,
71 percpu_ref_func_t
*release
);
72 void percpu_ref_cancel_init(struct percpu_ref
*ref
);
73 void percpu_ref_kill_and_confirm(struct percpu_ref
*ref
,
74 percpu_ref_func_t
*confirm_kill
);
77 * percpu_ref_kill - drop the initial ref
78 * @ref: percpu_ref to kill
80 * Must be used to drop the initial ref on a percpu refcount; must be called
81 * precisely once before shutdown.
83 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
84 * percpu counters and dropping the initial ref.
86 static inline void percpu_ref_kill(struct percpu_ref
*ref
)
88 return percpu_ref_kill_and_confirm(ref
, NULL
);
91 #define PCPU_STATUS_BITS 2
92 #define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1)
93 #define PCPU_REF_PTR 0
94 #define PCPU_REF_DEAD 1
96 #define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK)
99 * percpu_ref_get - increment a percpu refcount
100 * @ref: percpu_ref to get
102 * Analagous to atomic_inc().
104 static inline void percpu_ref_get(struct percpu_ref
*ref
)
106 unsigned __percpu
*pcpu_count
;
108 rcu_read_lock_sched();
110 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
112 if (likely(REF_STATUS(pcpu_count
) == PCPU_REF_PTR
))
113 __this_cpu_inc(*pcpu_count
);
115 atomic_inc(&ref
->count
);
117 rcu_read_unlock_sched();
121 * percpu_ref_tryget - try to increment a percpu refcount
122 * @ref: percpu_ref to try-get
124 * Increment a percpu refcount unless it has already been killed. Returns
125 * %true on success; %false on failure.
127 * Completion of percpu_ref_kill() in itself doesn't guarantee that tryget
128 * will fail. For such guarantee, percpu_ref_kill_and_confirm() should be
129 * used. After the confirm_kill callback is invoked, it's guaranteed that
130 * no new reference will be given out by percpu_ref_tryget().
132 static inline bool percpu_ref_tryget(struct percpu_ref
*ref
)
134 unsigned __percpu
*pcpu_count
;
137 rcu_read_lock_sched();
139 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
141 if (likely(REF_STATUS(pcpu_count
) == PCPU_REF_PTR
)) {
142 __this_cpu_inc(*pcpu_count
);
146 rcu_read_unlock_sched();
152 * percpu_ref_put - decrement a percpu refcount
153 * @ref: percpu_ref to put
155 * Decrement the refcount, and if 0, call the release function (which was passed
156 * to percpu_ref_init())
158 static inline void percpu_ref_put(struct percpu_ref
*ref
)
160 unsigned __percpu
*pcpu_count
;
162 rcu_read_lock_sched();
164 pcpu_count
= ACCESS_ONCE(ref
->pcpu_count
);
166 if (likely(REF_STATUS(pcpu_count
) == PCPU_REF_PTR
))
167 __this_cpu_dec(*pcpu_count
);
168 else if (unlikely(atomic_dec_and_test(&ref
->count
)))
171 rcu_read_unlock_sched();
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