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Linux/include/linux/percpu-refcount.h

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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_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 

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