1 /* SPDX-License-Identifier: GPL-2.0 */
2 /*
3  * Percpu refcounts:
4  * (C) 2012 Google, Inc.
5  * Author: Kent Overstreet <koverstreet@google.com>
6  *
7  * This implements a refcount with similar semantics to atomic_t - atomic_inc(),
8  * atomic_dec_and_test() - but percpu.
9  *
10  * There's one important difference between percpu refs and normal atomic_t
11  * refcounts; you have to keep track of your initial refcount, and then when you
12  * start shutting down you call percpu_ref_kill() _before_ dropping the initial
13  * refcount.
14  *
15  * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less
16  * than an atomic_t - this is because of the way shutdown works, see
17  * percpu_ref_kill()/PERCPU_COUNT_BIAS.
18  *
19  * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the
20  * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill()
21  * puts the ref back in single atomic_t mode, collecting the per cpu refs and
22  * issuing the appropriate barriers, and then marks the ref as shutting down so
23  * that percpu_ref_put() will check for the ref hitting 0.  After it returns,
24  * it's safe to drop the initial ref.
25  *
26  * USAGE:
27  *
28  * See fs/aio.c for some example usage; it's used there for struct kioctx, which
29  * is created when userspaces calls io_setup(), and destroyed when userspace
30  * calls io_destroy() or the process exits.
31  *
32  * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it
33  * removes the kioctx from the proccess's table of kioctxs and kills percpu_ref.
34  * After that, there can't be any new users of the kioctx (from lookup_ioctx())
35  * and it's then safe to drop the initial ref with percpu_ref_put().
36  *
37  * Note that the free path, free_ioctx(), needs to go through explicit call_rcu()
38  * to synchronize with RCU protected lookup_ioctx().  percpu_ref operations don't
39  * imply RCU grace periods of any kind and if a user wants to combine percpu_ref
40  * with RCU protection, it must be done explicitly.
41  *
42  * Code that does a two stage shutdown like this often needs some kind of
43  * explicit synchronization to ensure the initial refcount can only be dropped
44  * once - percpu_ref_kill() does this for you, it returns true once and false if
45  * someone else already called it. The aio code uses it this way, but it's not
46  * necessary if the code has some other mechanism to synchronize teardown.
47  * around.
48  */
49 
50 #ifndef _LINUX_PERCPU_REFCOUNT_H
51 #define _LINUX_PERCPU_REFCOUNT_H
52 
53 #include <linux/atomic.h>
54 #include <linux/percpu.h>
55 #include <linux/rcupdate.h>
56 #include <linux/types.h>
57 #include <linux/gfp.h>
58 
59 struct percpu_ref;
60 typedef void (percpu_ref_func_t)(struct percpu_ref *);
61 
62 /* flags set in the lower bits of percpu_ref->percpu_count_ptr */
63 enum {
64 	__PERCPU_REF_ATOMIC	= 1LU << 0,	/* operating in atomic mode */
65 	__PERCPU_REF_DEAD	= 1LU << 1,	/* (being) killed */
66 	__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
67 
68 	__PERCPU_REF_FLAG_BITS	= 2,
69 };
70 
71 /* @flags for percpu_ref_init() */
72 enum {
73 	/*
74 	 * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
75 	 * operation using percpu_ref_switch_to_percpu().  If initialized
76 	 * with this flag, the ref will stay in atomic mode until
77 	 * percpu_ref_switch_to_percpu() is invoked on it.
78 	 * Implies ALLOW_REINIT.
79 	 */
80 	PERCPU_REF_INIT_ATOMIC	= 1 << 0,
81 
82 	/*
83 	 * Start dead w/ ref == 0 in atomic mode.  Must be revived with
84 	 * percpu_ref_reinit() before used.  Implies INIT_ATOMIC and
85 	 * ALLOW_REINIT.
86 	 */
87 	PERCPU_REF_INIT_DEAD	= 1 << 1,
88 
89 	/*
90 	 * Allow switching from atomic mode to percpu mode.
91 	 */
92 	PERCPU_REF_ALLOW_REINIT	= 1 << 2,
93 };
94 
95 struct percpu_ref_data {
96 	atomic_long_t		count;
97 	percpu_ref_func_t	*release;
98 	percpu_ref_func_t	*confirm_switch;
99 	bool			force_atomic:1;
100 	bool			allow_reinit:1;
101 	struct rcu_head		rcu;
102 	struct percpu_ref	*ref;
103 };
104 
105 struct percpu_ref {
106 	/*
107 	 * The low bit of the pointer indicates whether the ref is in percpu
108 	 * mode; if set, then get/put will manipulate the atomic_t.
109 	 */
110 	unsigned long		percpu_count_ptr;
111 
112 	/*
113 	 * 'percpu_ref' is often embedded into user structure, and only
114 	 * 'percpu_count_ptr' is required in fast path, move other fields
115 	 * into 'percpu_ref_data', so we can reduce memory footprint in
116 	 * fast path.
117 	 */
118 	struct percpu_ref_data  *data;
119 };
120 
121 int __must_check percpu_ref_init(struct percpu_ref *ref,
122 				 percpu_ref_func_t *release, unsigned int flags,
123 				 gfp_t gfp);
124 void percpu_ref_exit(struct percpu_ref *ref);
125 void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
126 				 percpu_ref_func_t *confirm_switch);
127 void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
128 void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
129 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
130 				 percpu_ref_func_t *confirm_kill);
131 void percpu_ref_resurrect(struct percpu_ref *ref);
132 void percpu_ref_reinit(struct percpu_ref *ref);
133 bool percpu_ref_is_zero(struct percpu_ref *ref);
134 
135 /**
136  * percpu_ref_kill - drop the initial ref
137  * @ref: percpu_ref to kill
138  *
139  * Must be used to drop the initial ref on a percpu refcount; must be called
140  * precisely once before shutdown.
141  *
142  * Switches @ref into atomic mode before gathering up the percpu counters
143  * and dropping the initial ref.
144  *
145  * There are no implied RCU grace periods between kill and release.
146  */
147 static inline void percpu_ref_kill(struct percpu_ref *ref)
148 {
149 	percpu_ref_kill_and_confirm(ref, NULL);
150 }
151 
152 /*
153  * Internal helper.  Don't use outside percpu-refcount proper.  The
154  * function doesn't return the pointer and let the caller test it for NULL
155  * because doing so forces the compiler to generate two conditional
156  * branches as it can't assume that @ref->percpu_count is not NULL.
157  */
158 static inline bool __ref_is_percpu(struct percpu_ref *ref,
159 					  unsigned long __percpu **percpu_countp)
160 {
161 	unsigned long percpu_ptr;
162 
163 	/*
164 	 * The value of @ref->percpu_count_ptr is tested for
165 	 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
166 	 * used as a pointer.  If the compiler generates a separate fetch
167 	 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
168 	 * between contaminating the pointer value, meaning that
169 	 * READ_ONCE() is required when fetching it.
170 	 *
171 	 * The dependency ordering from the READ_ONCE() pairs
172 	 * with smp_store_release() in __percpu_ref_switch_to_percpu().
173 	 */
174 	percpu_ptr = READ_ONCE(ref->percpu_count_ptr);
175 
176 	/*
177 	 * Theoretically, the following could test just ATOMIC; however,
178 	 * then we'd have to mask off DEAD separately as DEAD may be
179 	 * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
180 	 * implies ATOMIC anyway.  Test them together.
181 	 */
182 	if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
183 		return false;
184 
185 	*percpu_countp = (unsigned long __percpu *)percpu_ptr;
186 	return true;
187 }
188 
189 /**
190  * percpu_ref_get_many - increment a percpu refcount
191  * @ref: percpu_ref to get
192  * @nr: number of references to get
193  *
194  * Analogous to atomic_long_add().
195  *
196  * This function is safe to call as long as @ref is between init and exit.
197  */
198 static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
199 {
200 	unsigned long __percpu *percpu_count;
201 
202 	rcu_read_lock();
203 
204 	if (__ref_is_percpu(ref, &percpu_count))
205 		this_cpu_add(*percpu_count, nr);
206 	else
207 		atomic_long_add(nr, &ref->data->count);
208 
209 	rcu_read_unlock();
210 }
211 
212 /**
213  * percpu_ref_get - increment a percpu refcount
214  * @ref: percpu_ref to get
215  *
216  * Analogous to atomic_long_inc().
217  *
218  * This function is safe to call as long as @ref is between init and exit.
219  */
220 static inline void percpu_ref_get(struct percpu_ref *ref)
221 {
222 	percpu_ref_get_many(ref, 1);
223 }
224 
225 /**
226  * percpu_ref_tryget_many - try to increment a percpu refcount
227  * @ref: percpu_ref to try-get
228  * @nr: number of references to get
229  *
230  * Increment a percpu refcount  by @nr unless its count already reached zero.
231  * Returns %true on success; %false on failure.
232  *
233  * This function is safe to call as long as @ref is between init and exit.
234  */
235 static inline bool percpu_ref_tryget_many(struct percpu_ref *ref,
236 					  unsigned long nr)
237 {
238 	unsigned long __percpu *percpu_count;
239 	bool ret;
240 
241 	rcu_read_lock();
242 
243 	if (__ref_is_percpu(ref, &percpu_count)) {
244 		this_cpu_add(*percpu_count, nr);
245 		ret = true;
246 	} else {
247 		ret = atomic_long_add_unless(&ref->data->count, nr, 0);
248 	}
249 
250 	rcu_read_unlock();
251 
252 	return ret;
253 }
254 
255 /**
256  * percpu_ref_tryget - try to increment a percpu refcount
257  * @ref: percpu_ref to try-get
258  *
259  * Increment a percpu refcount unless its count already reached zero.
260  * Returns %true on success; %false on failure.
261  *
262  * This function is safe to call as long as @ref is between init and exit.
263  */
264 static inline bool percpu_ref_tryget(struct percpu_ref *ref)
265 {
266 	return percpu_ref_tryget_many(ref, 1);
267 }
268 
269 /**
270  * percpu_ref_tryget_live_rcu - same as percpu_ref_tryget_live() but the
271  * caller is responsible for taking RCU.
272  *
273  * This function is safe to call as long as @ref is between init and exit.
274  */
275 static inline bool percpu_ref_tryget_live_rcu(struct percpu_ref *ref)
276 {
277 	unsigned long __percpu *percpu_count;
278 	bool ret = false;
279 
280 	WARN_ON_ONCE(!rcu_read_lock_held());
281 
282 	if (likely(__ref_is_percpu(ref, &percpu_count))) {
283 		this_cpu_inc(*percpu_count);
284 		ret = true;
285 	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
286 		ret = atomic_long_inc_not_zero(&ref->data->count);
287 	}
288 	return ret;
289 }
290 
291 /**
292  * percpu_ref_tryget_live - try to increment a live percpu refcount
293  * @ref: percpu_ref to try-get
294  *
295  * Increment a percpu refcount unless it has already been killed.  Returns
296  * %true on success; %false on failure.
297  *
298  * Completion of percpu_ref_kill() in itself doesn't guarantee that this
299  * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
300  * should be used.  After the confirm_kill callback is invoked, it's
301  * guaranteed that no new reference will be given out by
302  * percpu_ref_tryget_live().
303  *
304  * This function is safe to call as long as @ref is between init and exit.
305  */
306 static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
307 {
308 	bool ret = false;
309 
310 	rcu_read_lock();
311 	ret = percpu_ref_tryget_live_rcu(ref);
312 	rcu_read_unlock();
313 	return ret;
314 }
315 
316 /**
317  * percpu_ref_put_many - decrement a percpu refcount
318  * @ref: percpu_ref to put
319  * @nr: number of references to put
320  *
321  * Decrement the refcount, and if 0, call the release function (which was passed
322  * to percpu_ref_init())
323  *
324  * This function is safe to call as long as @ref is between init and exit.
325  */
326 static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
327 {
328 	unsigned long __percpu *percpu_count;
329 
330 	rcu_read_lock();
331 
332 	if (__ref_is_percpu(ref, &percpu_count))
333 		this_cpu_sub(*percpu_count, nr);
334 	else if (unlikely(atomic_long_sub_and_test(nr, &ref->data->count)))
335 		ref->data->release(ref);
336 
337 	rcu_read_unlock();
338 }
339 
340 /**
341  * percpu_ref_put - decrement a percpu refcount
342  * @ref: percpu_ref to put
343  *
344  * Decrement the refcount, and if 0, call the release function (which was passed
345  * to percpu_ref_init())
346  *
347  * This function is safe to call as long as @ref is between init and exit.
348  */
349 static inline void percpu_ref_put(struct percpu_ref *ref)
350 {
351 	percpu_ref_put_many(ref, 1);
352 }
353 
354 /**
355  * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
356  * @ref: percpu_ref to test
357  *
358  * Returns %true if @ref is dying or dead.
359  *
360  * This function is safe to call as long as @ref is between init and exit
361  * and the caller is responsible for synchronizing against state changes.
362  */
363 static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
364 {
365 	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
366 }
367 
368 #endif
369