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_func_t)(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_func_t	*release;
66 	percpu_ref_func_t	*confirm_kill;
67 	struct rcu_head		rcu;
68 };
69 
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);
75 
76 /**
77  * percpu_ref_kill - drop the initial ref
78  * @ref: percpu_ref to kill
79  *
80  * Must be used to drop the initial ref on a percpu refcount; must be called
81  * precisely once before shutdown.
82  *
83  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
84  * percpu counters and dropping the initial ref.
85  */
86 static inline void percpu_ref_kill(struct percpu_ref *ref)
87 {
88 	return percpu_ref_kill_and_confirm(ref, NULL);
89 }
90 
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
95 
96 #define REF_STATUS(count)	(((unsigned long) count) & PCPU_STATUS_MASK)
97 
98 /**
99  * percpu_ref_get - increment a percpu refcount
100  * @ref: percpu_ref to get
101  *
102  * Analagous to atomic_inc().
103   */
104 static inline void percpu_ref_get(struct percpu_ref *ref)
105 {
106 	unsigned __percpu *pcpu_count;
107 
108 	rcu_read_lock_sched();
109 
110 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
111 
112 	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
113 		__this_cpu_inc(*pcpu_count);
114 	else
115 		atomic_inc(&ref->count);
116 
117 	rcu_read_unlock_sched();
118 }
119 
120 /**
121  * percpu_ref_tryget - try to increment a percpu refcount
122  * @ref: percpu_ref to try-get
123  *
124  * Increment a percpu refcount unless it has already been killed.  Returns
125  * %true on success; %false on failure.
126  *
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().
131  */
132 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
133 {
134 	unsigned __percpu *pcpu_count;
135 	int ret = false;
136 
137 	rcu_read_lock_sched();
138 
139 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
140 
141 	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
142 		__this_cpu_inc(*pcpu_count);
143 		ret = true;
144 	}
145 
146 	rcu_read_unlock_sched();
147 
148 	return ret;
149 }
150 
151 /**
152  * percpu_ref_put - decrement a percpu refcount
153  * @ref: percpu_ref to put
154  *
155  * Decrement the refcount, and if 0, call the release function (which was passed
156  * to percpu_ref_init())
157  */
158 static inline void percpu_ref_put(struct percpu_ref *ref)
159 {
160 	unsigned __percpu *pcpu_count;
161 
162 	rcu_read_lock_sched();
163 
164 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
165 
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)))
169 		ref->release(ref);
170 
171 	rcu_read_unlock_sched();
172 }
173 
174 #endif
175