percpu-refcount.h

  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
 59struct percpu_ref;
 60typedef void (percpu_ref_func_t)(struct percpu_ref *);
 61
 62/* flags set in the lower bits of percpu_ref->percpu_count_ptr */
 63enum {
 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() */
 72enum {
 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
 95struct 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
105struct 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
121int __must_check percpu_ref_init(struct percpu_ref *ref,
122				 percpu_ref_func_t *release, unsigned int flags,
123				 gfp_t gfp);
124void percpu_ref_exit(struct percpu_ref *ref);
125void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
126				 percpu_ref_func_t *confirm_switch);
127void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref);
128void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
129void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
130				 percpu_ref_func_t *confirm_kill);
131void percpu_ref_resurrect(struct percpu_ref *ref);
132void percpu_ref_reinit(struct percpu_ref *ref);
133bool 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 */
147static 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 */
158static 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 */
198static 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 */
220static 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 */
235static 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 */
264static 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 */
275static 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 */
306static 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 */
326static 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 */
349static 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 */
363static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
364{
365	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
366}
367
368#endif