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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()/PERCPU_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 synchronize_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 #include <linux/gfp.h>
 53 
 54 struct percpu_ref;
 55 typedef void (percpu_ref_func_t)(struct percpu_ref *);
 56 
 57 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
 58 enum {
 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,
 62 
 63         __PERCPU_REF_FLAG_BITS  = 2,
 64 };
 65 
 66 /* @flags for percpu_ref_init() */
 67 enum {
 68         /*
 69          * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
 70          * operation using percpu_ref_switch_to_percpu().  If initialized
 71          * with this flag, the ref will stay in atomic mode until
 72          * percpu_ref_switch_to_percpu() is invoked on it.
 73          */
 74         PERCPU_REF_INIT_ATOMIC  = 1 << 0,
 75 
 76         /*
 77          * Start dead w/ ref == 0 in atomic mode.  Must be revived with
 78          * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
 79          */
 80         PERCPU_REF_INIT_DEAD    = 1 << 1,
 81 };
 82 
 83 struct percpu_ref {
 84         atomic_long_t           count;
 85         /*
 86          * The low bit of the pointer indicates whether the ref is in percpu
 87          * mode; if set, then get/put will manipulate the atomic_t.
 88          */
 89         unsigned long           percpu_count_ptr;
 90         percpu_ref_func_t       *release;
 91         percpu_ref_func_t       *confirm_switch;
 92         bool                    force_atomic:1;
 93         struct rcu_head         rcu;
 94 };
 95 
 96 int __must_check percpu_ref_init(struct percpu_ref *ref,
 97                                  percpu_ref_func_t *release, unsigned int flags,
 98                                  gfp_t gfp);
 99 void percpu_ref_exit(struct percpu_ref *ref);
100 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
101                                  percpu_ref_func_t *confirm_switch);
102 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
103 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
104                                  percpu_ref_func_t *confirm_kill);
105 void percpu_ref_reinit(struct percpu_ref *ref);
106 
107 /**
108  * percpu_ref_kill - drop the initial ref
109  * @ref: percpu_ref to kill
110  *
111  * Must be used to drop the initial ref on a percpu refcount; must be called
112  * precisely once before shutdown.
113  *
114  * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the
115  * percpu counters and dropping the initial ref.
116  */
117 static inline void percpu_ref_kill(struct percpu_ref *ref)
118 {
119         percpu_ref_kill_and_confirm(ref, NULL);
120 }
121 
122 /*
123  * Internal helper.  Don't use outside percpu-refcount proper.  The
124  * function doesn't return the pointer and let the caller test it for NULL
125  * because doing so forces the compiler to generate two conditional
126  * branches as it can't assume that @ref->percpu_count is not NULL.
127  */
128 static inline bool __ref_is_percpu(struct percpu_ref *ref,
129                                           unsigned long __percpu **percpu_countp)
130 {
131         unsigned long percpu_ptr;
132 
133         /*
134          * The value of @ref->percpu_count_ptr is tested for
135          * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
136          * used as a pointer.  If the compiler generates a separate fetch
137          * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
138          * between contaminating the pointer value, meaning that
139          * ACCESS_ONCE() is required when fetching it.
140          *
141          * Also, we need a data dependency barrier to be paired with
142          * smp_store_release() in __percpu_ref_switch_to_percpu().
143          *
144          * Use lockless deref which contains both.
145          */
146         percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
147 
148         /*
149          * Theoretically, the following could test just ATOMIC; however,
150          * then we'd have to mask off DEAD separately as DEAD may be
151          * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
152          * implies ATOMIC anyway.  Test them together.
153          */
154         if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
155                 return false;
156 
157         *percpu_countp = (unsigned long __percpu *)percpu_ptr;
158         return true;
159 }
160 
161 /**
162  * percpu_ref_get_many - increment a percpu refcount
163  * @ref: percpu_ref to get
164  * @nr: number of references to get
165  *
166  * Analogous to atomic_long_add().
167  *
168  * This function is safe to call as long as @ref is between init and exit.
169  */
170 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
171 {
172         unsigned long __percpu *percpu_count;
173 
174         rcu_read_lock_sched();
175 
176         if (__ref_is_percpu(ref, &percpu_count))
177                 this_cpu_add(*percpu_count, nr);
178         else
179                 atomic_long_add(nr, &ref->count);
180 
181         rcu_read_unlock_sched();
182 }
183 
184 /**
185  * percpu_ref_get - increment a percpu refcount
186  * @ref: percpu_ref to get
187  *
188  * Analagous to atomic_long_inc().
189  *
190  * This function is safe to call as long as @ref is between init and exit.
191  */
192 static inline void percpu_ref_get(struct percpu_ref *ref)
193 {
194         percpu_ref_get_many(ref, 1);
195 }
196 
197 /**
198  * percpu_ref_tryget - try to increment a percpu refcount
199  * @ref: percpu_ref to try-get
200  *
201  * Increment a percpu refcount unless its count already reached zero.
202  * Returns %true on success; %false on failure.
203  *
204  * This function is safe to call as long as @ref is between init and exit.
205  */
206 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
207 {
208         unsigned long __percpu *percpu_count;
209         int ret;
210 
211         rcu_read_lock_sched();
212 
213         if (__ref_is_percpu(ref, &percpu_count)) {
214                 this_cpu_inc(*percpu_count);
215                 ret = true;
216         } else {
217                 ret = atomic_long_inc_not_zero(&ref->count);
218         }
219 
220         rcu_read_unlock_sched();
221 
222         return ret;
223 }
224 
225 /**
226  * percpu_ref_tryget_live - try to increment a live percpu refcount
227  * @ref: percpu_ref to try-get
228  *
229  * Increment a percpu refcount unless it has already been killed.  Returns
230  * %true on success; %false on failure.
231  *
232  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
233  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
234  * should be used.  After the confirm_kill callback is invoked, it's
235  * guaranteed that no new reference will be given out by
236  * percpu_ref_tryget_live().
237  *
238  * This function is safe to call as long as @ref is between init and exit.
239  */
240 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
241 {
242         unsigned long __percpu *percpu_count;
243         int ret = false;
244 
245         rcu_read_lock_sched();
246 
247         if (__ref_is_percpu(ref, &percpu_count)) {
248                 this_cpu_inc(*percpu_count);
249                 ret = true;
250         } else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
251                 ret = atomic_long_inc_not_zero(&ref->count);
252         }
253 
254         rcu_read_unlock_sched();
255 
256         return ret;
257 }
258 
259 /**
260  * percpu_ref_put_many - decrement a percpu refcount
261  * @ref: percpu_ref to put
262  * @nr: number of references to put
263  *
264  * Decrement the refcount, and if 0, call the release function (which was passed
265  * to percpu_ref_init())
266  *
267  * This function is safe to call as long as @ref is between init and exit.
268  */
269 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
270 {
271         unsigned long __percpu *percpu_count;
272 
273         rcu_read_lock_sched();
274 
275         if (__ref_is_percpu(ref, &percpu_count))
276                 this_cpu_sub(*percpu_count, nr);
277         else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
278                 ref->release(ref);
279 
280         rcu_read_unlock_sched();
281 }
282 
283 /**
284  * percpu_ref_put - decrement a percpu refcount
285  * @ref: percpu_ref to put
286  *
287  * Decrement the refcount, and if 0, call the release function (which was passed
288  * to percpu_ref_init())
289  *
290  * This function is safe to call as long as @ref is between init and exit.
291  */
292 static inline void percpu_ref_put(struct percpu_ref *ref)
293 {
294         percpu_ref_put_many(ref, 1);
295 }
296 
297 /**
298  * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
299  * @ref: percpu_ref to test
300  *
301  * Returns %true if @ref is dying or dead.
302  *
303  * This function is safe to call as long as @ref is between init and exit
304  * and the caller is responsible for synchronizing against state changes.
305  */
306 static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
307 {
308         return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
309 }
310 
311 /**
312  * percpu_ref_is_zero - test whether a percpu refcount reached zero
313  * @ref: percpu_ref to test
314  *
315  * Returns %true if @ref reached zero.
316  *
317  * This function is safe to call as long as @ref is between init and exit.
318  */
319 static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
320 {
321         unsigned long __percpu *percpu_count;
322 
323         if (__ref_is_percpu(ref, &percpu_count))
324                 return false;
325         return !atomic_long_read(&ref->count);
326 }
327 
328 #endif
329 

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