Line data Source code
1 : #define pr_fmt(fmt) "%s: " fmt "\n", __func__
2 :
3 : #include <linux/kernel.h>
4 : #include <linux/sched.h>
5 : #include <linux/wait.h>
6 : #include <linux/percpu-refcount.h>
7 :
8 : /*
9 : * Initially, a percpu refcount is just a set of percpu counters. Initially, we
10 : * don't try to detect the ref hitting 0 - which means that get/put can just
11 : * increment or decrement the local counter. Note that the counter on a
12 : * particular cpu can (and will) wrap - this is fine, when we go to shutdown the
13 : * percpu counters will all sum to the correct value
14 : *
15 : * (More precisely: because moduler arithmatic is commutative the sum of all the
16 : * percpu_count vars will be equal to what it would have been if all the gets
17 : * and puts were done to a single integer, even if some of the percpu integers
18 : * overflow or underflow).
19 : *
20 : * The real trick to implementing percpu refcounts is shutdown. We can't detect
21 : * the ref hitting 0 on every put - this would require global synchronization
22 : * and defeat the whole purpose of using percpu refs.
23 : *
24 : * What we do is require the user to keep track of the initial refcount; we know
25 : * the ref can't hit 0 before the user drops the initial ref, so as long as we
26 : * convert to non percpu mode before the initial ref is dropped everything
27 : * works.
28 : *
29 : * Converting to non percpu mode is done with some RCUish stuff in
30 : * percpu_ref_kill. Additionally, we need a bias value so that the
31 : * atomic_long_t can't hit 0 before we've added up all the percpu refs.
32 : */
33 :
34 : #define PERCPU_COUNT_BIAS (1LU << (BITS_PER_LONG - 1))
35 :
36 : static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq);
37 :
38 : static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref)
39 : {
40 0 : return (unsigned long __percpu *)
41 0 : (ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);
42 : }
43 :
44 : /**
45 : * percpu_ref_init - initialize a percpu refcount
46 : * @ref: percpu_ref to initialize
47 : * @release: function which will be called when refcount hits 0
48 : * @flags: PERCPU_REF_INIT_* flags
49 : * @gfp: allocation mask to use
50 : *
51 : * Initializes @ref. If @flags is zero, @ref starts in percpu mode with a
52 : * refcount of 1; analagous to atomic_long_set(ref, 1). See the
53 : * definitions of PERCPU_REF_INIT_* flags for flag behaviors.
54 : *
55 : * Note that @release must not sleep - it may potentially be called from RCU
56 : * callback context by percpu_ref_kill().
57 : */
58 1 : int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,
59 : unsigned int flags, gfp_t gfp)
60 : {
61 : size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,
62 : __alignof__(unsigned long));
63 : unsigned long start_count = 0;
64 :
65 1 : ref->percpu_count_ptr = (unsigned long)
66 1 : __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
67 1 : if (!ref->percpu_count_ptr)
68 : return -ENOMEM;
69 :
70 1 : ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
71 :
72 1 : if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))
73 0 : ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
74 : else
75 : start_count += PERCPU_COUNT_BIAS;
76 :
77 1 : if (flags & PERCPU_REF_INIT_DEAD)
78 0 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
79 : else
80 1 : start_count++;
81 :
82 1 : atomic_long_set(&ref->count, start_count);
83 :
84 1 : ref->release = release;
85 1 : return 0;
86 : }
87 : EXPORT_SYMBOL_GPL(percpu_ref_init);
88 :
89 : /**
90 : * percpu_ref_exit - undo percpu_ref_init()
91 : * @ref: percpu_ref to exit
92 : *
93 : * This function exits @ref. The caller is responsible for ensuring that
94 : * @ref is no longer in active use. The usual places to invoke this
95 : * function from are the @ref->release() callback or in init failure path
96 : * where percpu_ref_init() succeeded but other parts of the initialization
97 : * of the embedding object failed.
98 : */
99 0 : void percpu_ref_exit(struct percpu_ref *ref)
100 : {
101 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
102 :
103 0 : if (percpu_count) {
104 0 : free_percpu(percpu_count);
105 0 : ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
106 : }
107 0 : }
108 : EXPORT_SYMBOL_GPL(percpu_ref_exit);
109 :
110 0 : static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
111 : {
112 0 : struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
113 :
114 0 : ref->confirm_switch(ref);
115 0 : ref->confirm_switch = NULL;
116 0 : wake_up_all(&percpu_ref_switch_waitq);
117 :
118 : /* drop ref from percpu_ref_switch_to_atomic() */
119 : percpu_ref_put(ref);
120 0 : }
121 :
122 0 : static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
123 : {
124 0 : struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);
125 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
126 : unsigned long count = 0;
127 : int cpu;
128 :
129 0 : for_each_possible_cpu(cpu)
130 0 : count += *per_cpu_ptr(percpu_count, cpu);
131 :
132 : pr_debug("global %ld percpu %ld",
133 : atomic_long_read(&ref->count), (long)count);
134 :
135 : /*
136 : * It's crucial that we sum the percpu counters _before_ adding the sum
137 : * to &ref->count; since gets could be happening on one cpu while puts
138 : * happen on another, adding a single cpu's count could cause
139 : * @ref->count to hit 0 before we've got a consistent value - but the
140 : * sum of all the counts will be consistent and correct.
141 : *
142 : * Subtracting the bias value then has to happen _after_ adding count to
143 : * &ref->count; we need the bias value to prevent &ref->count from
144 : * reaching 0 before we add the percpu counts. But doing it at the same
145 : * time is equivalent and saves us atomic operations:
146 : */
147 0 : atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count);
148 :
149 : WARN_ONCE(atomic_long_read(&ref->count) <= 0,
150 : "percpu ref (%pf) <= 0 (%ld) after switching to atomic",
151 : ref->release, atomic_long_read(&ref->count));
152 :
153 : /* @ref is viewed as dead on all CPUs, send out switch confirmation */
154 0 : percpu_ref_call_confirm_rcu(rcu);
155 0 : }
156 :
157 0 : static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref)
158 : {
159 0 : }
160 :
161 0 : static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
162 : percpu_ref_func_t *confirm_switch)
163 : {
164 0 : if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC)) {
165 : /* switching from percpu to atomic */
166 0 : ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
167 :
168 : /*
169 : * Non-NULL ->confirm_switch is used to indicate that
170 : * switching is in progress. Use noop one if unspecified.
171 : */
172 : WARN_ON_ONCE(ref->confirm_switch);
173 0 : ref->confirm_switch =
174 0 : confirm_switch ?: percpu_ref_noop_confirm_switch;
175 :
176 : percpu_ref_get(ref); /* put after confirmation */
177 0 : call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);
178 0 : } else if (confirm_switch) {
179 : /*
180 : * Somebody already set ATOMIC. Switching may still be in
181 : * progress. @confirm_switch must be invoked after the
182 : * switching is complete and a full sched RCU grace period
183 : * has passed. Wait synchronously for the previous
184 : * switching and schedule @confirm_switch invocation.
185 : */
186 0 : wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
187 0 : ref->confirm_switch = confirm_switch;
188 :
189 : percpu_ref_get(ref); /* put after confirmation */
190 0 : call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu);
191 : }
192 0 : }
193 :
194 : /**
195 : * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode
196 : * @ref: percpu_ref to switch to atomic mode
197 : * @confirm_switch: optional confirmation callback
198 : *
199 : * There's no reason to use this function for the usual reference counting.
200 : * Use percpu_ref_kill[_and_confirm]().
201 : *
202 : * Schedule switching of @ref to atomic mode. All its percpu counts will
203 : * be collected to the main atomic counter. On completion, when all CPUs
204 : * are guaraneed to be in atomic mode, @confirm_switch, which may not
205 : * block, is invoked. This function may be invoked concurrently with all
206 : * the get/put operations and can safely be mixed with kill and reinit
207 : * operations. Note that @ref will stay in atomic mode across kill/reinit
208 : * cycles until percpu_ref_switch_to_percpu() is called.
209 : *
210 : * This function normally doesn't block and can be called from any context
211 : * but it may block if @confirm_kill is specified and @ref is already in
212 : * the process of switching to atomic mode. In such cases, @confirm_switch
213 : * will be invoked after the switching is complete.
214 : *
215 : * Due to the way percpu_ref is implemented, @confirm_switch will be called
216 : * after at least one full sched RCU grace period has passed but this is an
217 : * implementation detail and must not be depended upon.
218 : */
219 0 : void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
220 : percpu_ref_func_t *confirm_switch)
221 : {
222 0 : ref->force_atomic = true;
223 0 : __percpu_ref_switch_to_atomic(ref, confirm_switch);
224 0 : }
225 :
226 0 : static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)
227 : {
228 : unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
229 : int cpu;
230 :
231 : BUG_ON(!percpu_count);
232 :
233 0 : if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))
234 0 : return;
235 :
236 0 : wait_event(percpu_ref_switch_waitq, !ref->confirm_switch);
237 :
238 0 : atomic_long_add(PERCPU_COUNT_BIAS, &ref->count);
239 :
240 : /*
241 : * Restore per-cpu operation. smp_store_release() is paired with
242 : * smp_read_barrier_depends() in __ref_is_percpu() and guarantees
243 : * that the zeroing is visible to all percpu accesses which can see
244 : * the following __PERCPU_REF_ATOMIC clearing.
245 : */
246 0 : for_each_possible_cpu(cpu)
247 0 : *per_cpu_ptr(percpu_count, cpu) = 0;
248 :
249 0 : smp_store_release(&ref->percpu_count_ptr,
250 : ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);
251 : }
252 :
253 : /**
254 : * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode
255 : * @ref: percpu_ref to switch to percpu mode
256 : *
257 : * There's no reason to use this function for the usual reference counting.
258 : * To re-use an expired ref, use percpu_ref_reinit().
259 : *
260 : * Switch @ref to percpu mode. This function may be invoked concurrently
261 : * with all the get/put operations and can safely be mixed with kill and
262 : * reinit operations. This function reverses the sticky atomic state set
263 : * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic(). If @ref is
264 : * dying or dead, the actual switching takes place on the following
265 : * percpu_ref_reinit().
266 : *
267 : * This function normally doesn't block and can be called from any context
268 : * but it may block if @ref is in the process of switching to atomic mode
269 : * by percpu_ref_switch_atomic().
270 : */
271 0 : void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
272 : {
273 0 : ref->force_atomic = false;
274 :
275 : /* a dying or dead ref can't be switched to percpu mode w/o reinit */
276 0 : if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD))
277 0 : __percpu_ref_switch_to_percpu(ref);
278 0 : }
279 :
280 : /**
281 : * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation
282 : * @ref: percpu_ref to kill
283 : * @confirm_kill: optional confirmation callback
284 : *
285 : * Equivalent to percpu_ref_kill() but also schedules kill confirmation if
286 : * @confirm_kill is not NULL. @confirm_kill, which may not block, will be
287 : * called after @ref is seen as dead from all CPUs at which point all
288 : * further invocations of percpu_ref_tryget_live() will fail. See
289 : * percpu_ref_tryget_live() for details.
290 : *
291 : * This function normally doesn't block and can be called from any context
292 : * but it may block if @confirm_kill is specified and @ref is in the
293 : * process of switching to atomic mode by percpu_ref_switch_atomic().
294 : *
295 : * Due to the way percpu_ref is implemented, @confirm_switch will be called
296 : * after at least one full sched RCU grace period has passed but this is an
297 : * implementation detail and must not be depended upon.
298 : */
299 0 : void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
300 : percpu_ref_func_t *confirm_kill)
301 : {
302 : WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,
303 : "%s called more than once on %pf!", __func__, ref->release);
304 :
305 0 : ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
306 0 : __percpu_ref_switch_to_atomic(ref, confirm_kill);
307 : percpu_ref_put(ref);
308 0 : }
309 : EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
310 :
311 : /**
312 : * percpu_ref_reinit - re-initialize a percpu refcount
313 : * @ref: perpcu_ref to re-initialize
314 : *
315 : * Re-initialize @ref so that it's in the same state as when it finished
316 : * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD. @ref must have been
317 : * initialized successfully and reached 0 but not exited.
318 : *
319 : * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
320 : * this function is in progress.
321 : */
322 0 : void percpu_ref_reinit(struct percpu_ref *ref)
323 : {
324 : WARN_ON_ONCE(!percpu_ref_is_zero(ref));
325 :
326 0 : ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
327 : percpu_ref_get(ref);
328 0 : if (!ref->force_atomic)
329 0 : __percpu_ref_switch_to_percpu(ref);
330 0 : }
331 : EXPORT_SYMBOL_GPL(percpu_ref_reinit);
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