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()/PERCPU_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>
52 #include <linux/gfp.h>
55 typedef void (percpu_ref_func_t)(struct percpu_ref *);
57 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
59 __PERCPU_REF_ATOMIC = 1LU << 0, /* operating in atomic mode */
60 __PERCPU_REF_DEAD = 1LU << 1, /* (being) killed */
61 __PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
63 __PERCPU_REF_FLAG_BITS = 2,
69 * The low bit of the pointer indicates whether the ref is in percpu
70 * mode; if set, then get/put will manipulate the atomic_t.
72 unsigned long percpu_count_ptr;
73 percpu_ref_func_t *release;
74 percpu_ref_func_t *confirm_switch;
78 int __must_check percpu_ref_init(struct percpu_ref *ref,
79 percpu_ref_func_t *release, gfp_t gfp);
80 void percpu_ref_exit(struct percpu_ref *ref);
81 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
82 percpu_ref_func_t *confirm_kill);
83 void percpu_ref_reinit(struct percpu_ref *ref);
86 * percpu_ref_kill - drop the initial ref
87 * @ref: percpu_ref to kill
89 * Must be used to drop the initial ref on a percpu refcount; must be called
90 * precisely once before shutdown.
92 * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
93 * percpu counters and dropping the initial ref.
95 static inline void percpu_ref_kill(struct percpu_ref *ref)
97 return percpu_ref_kill_and_confirm(ref, NULL);
101 * Internal helper. Don't use outside percpu-refcount proper. The
102 * function doesn't return the pointer and let the caller test it for NULL
103 * because doing so forces the compiler to generate two conditional
104 * branches as it can't assume that @ref->percpu_count is not NULL.
106 static inline bool __ref_is_percpu(struct percpu_ref *ref,
107 unsigned long __percpu **percpu_countp)
109 unsigned long percpu_ptr = ACCESS_ONCE(ref->percpu_count_ptr);
111 /* paired with smp_store_release() in percpu_ref_reinit() */
112 smp_read_barrier_depends();
114 if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
117 *percpu_countp = (unsigned long __percpu *)percpu_ptr;
122 * percpu_ref_get - increment a percpu refcount
123 * @ref: percpu_ref to get
125 * Analagous to atomic_long_inc().
127 * This function is safe to call as long as @ref is between init and exit.
129 static inline void percpu_ref_get(struct percpu_ref *ref)
131 unsigned long __percpu *percpu_count;
133 rcu_read_lock_sched();
135 if (__ref_is_percpu(ref, &percpu_count))
136 this_cpu_inc(*percpu_count);
138 atomic_long_inc(&ref->count);
140 rcu_read_unlock_sched();
144 * percpu_ref_tryget - try to increment a percpu refcount
145 * @ref: percpu_ref to try-get
147 * Increment a percpu refcount unless its count already reached zero.
148 * Returns %true on success; %false on failure.
150 * This function is safe to call as long as @ref is between init and exit.
152 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
154 unsigned long __percpu *percpu_count;
157 rcu_read_lock_sched();
159 if (__ref_is_percpu(ref, &percpu_count)) {
160 this_cpu_inc(*percpu_count);
163 ret = atomic_long_inc_not_zero(&ref->count);
166 rcu_read_unlock_sched();
172 * percpu_ref_tryget_live - try to increment a live percpu refcount
173 * @ref: percpu_ref to try-get
175 * Increment a percpu refcount unless it has already been killed. Returns
176 * %true on success; %false on failure.
178 * Completion of percpu_ref_kill() in itself doesn't guarantee that this
179 * function will fail. For such guarantee, percpu_ref_kill_and_confirm()
180 * should be used. After the confirm_kill callback is invoked, it's
181 * guaranteed that no new reference will be given out by
182 * percpu_ref_tryget_live().
184 * This function is safe to call as long as @ref is between init and exit.
186 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
188 unsigned long __percpu *percpu_count;
191 rcu_read_lock_sched();
193 if (__ref_is_percpu(ref, &percpu_count)) {
194 this_cpu_inc(*percpu_count);
198 rcu_read_unlock_sched();
204 * percpu_ref_put - decrement a percpu refcount
205 * @ref: percpu_ref to put
207 * Decrement the refcount, and if 0, call the release function (which was passed
208 * to percpu_ref_init())
210 * This function is safe to call as long as @ref is between init and exit.
212 static inline void percpu_ref_put(struct percpu_ref *ref)
214 unsigned long __percpu *percpu_count;
216 rcu_read_lock_sched();
218 if (__ref_is_percpu(ref, &percpu_count))
219 this_cpu_dec(*percpu_count);
220 else if (unlikely(atomic_long_dec_and_test(&ref->count)))
223 rcu_read_unlock_sched();
227 * percpu_ref_is_zero - test whether a percpu refcount reached zero
228 * @ref: percpu_ref to test
230 * Returns %true if @ref reached zero.
232 * This function is safe to call as long as @ref is between init and exit.
234 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
236 unsigned long __percpu *percpu_count;
238 if (__ref_is_percpu(ref, &percpu_count))
240 return !atomic_long_read(&ref->count);