Root/kernel/srcu.c

1/*
2 * Sleepable Read-Copy Update mechanism for mutual exclusion.
3 *
4 * This program is free software; you can redistribute it and/or modify
5 * it under the terms of the GNU General Public License as published by
6 * the Free Software Foundation; either version 2 of the License, or
7 * (at your option) any later version.
8 *
9 * This program is distributed in the hope that it will be useful,
10 * but WITHOUT ANY WARRANTY; without even the implied warranty of
11 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 * GNU General Public License for more details.
13 *
14 * You should have received a copy of the GNU General Public License
15 * along with this program; if not, write to the Free Software
16 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
17 *
18 * Copyright (C) IBM Corporation, 2006
19 *
20 * Author: Paul McKenney <paulmck@us.ibm.com>
21 *
22 * For detailed explanation of Read-Copy Update mechanism see -
23 * Documentation/RCU/ *.txt
24 *
25 */
26
27#include <linux/module.h>
28#include <linux/mutex.h>
29#include <linux/percpu.h>
30#include <linux/preempt.h>
31#include <linux/rcupdate.h>
32#include <linux/sched.h>
33#include <linux/smp.h>
34#include <linux/srcu.h>
35
36static int init_srcu_struct_fields(struct srcu_struct *sp)
37{
38    sp->completed = 0;
39    mutex_init(&sp->mutex);
40    sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
41    return sp->per_cpu_ref ? 0 : -ENOMEM;
42}
43
44#ifdef CONFIG_DEBUG_LOCK_ALLOC
45
46int __init_srcu_struct(struct srcu_struct *sp, const char *name,
47               struct lock_class_key *key)
48{
49#ifdef CONFIG_DEBUG_LOCK_ALLOC
50    /* Don't re-initialize a lock while it is held. */
51    debug_check_no_locks_freed((void *)sp, sizeof(*sp));
52    lockdep_init_map(&sp->dep_map, name, key, 0);
53#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
54    return init_srcu_struct_fields(sp);
55}
56EXPORT_SYMBOL_GPL(__init_srcu_struct);
57
58#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
59
60/**
61 * init_srcu_struct - initialize a sleep-RCU structure
62 * @sp: structure to initialize.
63 *
64 * Must invoke this on a given srcu_struct before passing that srcu_struct
65 * to any other function. Each srcu_struct represents a separate domain
66 * of SRCU protection.
67 */
68int init_srcu_struct(struct srcu_struct *sp)
69{
70    return init_srcu_struct_fields(sp);
71}
72EXPORT_SYMBOL_GPL(init_srcu_struct);
73
74#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
75
76/*
77 * srcu_readers_active_idx -- returns approximate number of readers
78 * active on the specified rank of per-CPU counters.
79 */
80
81static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
82{
83    int cpu;
84    int sum;
85
86    sum = 0;
87    for_each_possible_cpu(cpu)
88        sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
89    return sum;
90}
91
92/**
93 * srcu_readers_active - returns approximate number of readers.
94 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
95 *
96 * Note that this is not an atomic primitive, and can therefore suffer
97 * severe errors when invoked on an active srcu_struct. That said, it
98 * can be useful as an error check at cleanup time.
99 */
100static int srcu_readers_active(struct srcu_struct *sp)
101{
102    return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
103}
104
105/**
106 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
107 * @sp: structure to clean up.
108 *
109 * Must invoke this after you are finished using a given srcu_struct that
110 * was initialized via init_srcu_struct(), else you leak memory.
111 */
112void cleanup_srcu_struct(struct srcu_struct *sp)
113{
114    int sum;
115
116    sum = srcu_readers_active(sp);
117    WARN_ON(sum); /* Leakage unless caller handles error. */
118    if (sum != 0)
119        return;
120    free_percpu(sp->per_cpu_ref);
121    sp->per_cpu_ref = NULL;
122}
123EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
124
125/*
126 * Counts the new reader in the appropriate per-CPU element of the
127 * srcu_struct. Must be called from process context.
128 * Returns an index that must be passed to the matching srcu_read_unlock().
129 */
130int __srcu_read_lock(struct srcu_struct *sp)
131{
132    int idx;
133
134    preempt_disable();
135    idx = sp->completed & 0x1;
136    barrier(); /* ensure compiler looks -once- at sp->completed. */
137    per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
138    srcu_barrier(); /* ensure compiler won't misorder critical section. */
139    preempt_enable();
140    return idx;
141}
142EXPORT_SYMBOL_GPL(__srcu_read_lock);
143
144/*
145 * Removes the count for the old reader from the appropriate per-CPU
146 * element of the srcu_struct. Note that this may well be a different
147 * CPU than that which was incremented by the corresponding srcu_read_lock().
148 * Must be called from process context.
149 */
150void __srcu_read_unlock(struct srcu_struct *sp, int idx)
151{
152    preempt_disable();
153    srcu_barrier(); /* ensure compiler won't misorder critical section. */
154    per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
155    preempt_enable();
156}
157EXPORT_SYMBOL_GPL(__srcu_read_unlock);
158
159/*
160 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
161 */
162static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
163{
164    int idx;
165
166    idx = sp->completed;
167    mutex_lock(&sp->mutex);
168
169    /*
170     * Check to see if someone else did the work for us while we were
171     * waiting to acquire the lock. We need -two- advances of
172     * the counter, not just one. If there was but one, we might have
173     * shown up -after- our helper's first synchronize_sched(), thus
174     * having failed to prevent CPU-reordering races with concurrent
175     * srcu_read_unlock()s on other CPUs (see comment below). So we
176     * either (1) wait for two or (2) supply the second ourselves.
177     */
178
179    if ((sp->completed - idx) >= 2) {
180        mutex_unlock(&sp->mutex);
181        return;
182    }
183
184    sync_func(); /* Force memory barrier on all CPUs. */
185
186    /*
187     * The preceding synchronize_sched() ensures that any CPU that
188     * sees the new value of sp->completed will also see any preceding
189     * changes to data structures made by this CPU. This prevents
190     * some other CPU from reordering the accesses in its SRCU
191     * read-side critical section to precede the corresponding
192     * srcu_read_lock() -- ensuring that such references will in
193     * fact be protected.
194     *
195     * So it is now safe to do the flip.
196     */
197
198    idx = sp->completed & 0x1;
199    sp->completed++;
200
201    sync_func(); /* Force memory barrier on all CPUs. */
202
203    /*
204     * At this point, because of the preceding synchronize_sched(),
205     * all srcu_read_lock() calls using the old counters have completed.
206     * Their corresponding critical sections might well be still
207     * executing, but the srcu_read_lock() primitives themselves
208     * will have finished executing.
209     */
210
211    while (srcu_readers_active_idx(sp, idx))
212        schedule_timeout_interruptible(1);
213
214    sync_func(); /* Force memory barrier on all CPUs. */
215
216    /*
217     * The preceding synchronize_sched() forces all srcu_read_unlock()
218     * primitives that were executing concurrently with the preceding
219     * for_each_possible_cpu() loop to have completed by this point.
220     * More importantly, it also forces the corresponding SRCU read-side
221     * critical sections to have also completed, and the corresponding
222     * references to SRCU-protected data items to be dropped.
223     *
224     * Note:
225     *
226     * Despite what you might think at first glance, the
227     * preceding synchronize_sched() -must- be within the
228     * critical section ended by the following mutex_unlock().
229     * Otherwise, a task taking the early exit can race
230     * with a srcu_read_unlock(), which might have executed
231     * just before the preceding srcu_readers_active() check,
232     * and whose CPU might have reordered the srcu_read_unlock()
233     * with the preceding critical section. In this case, there
234     * is nothing preventing the synchronize_sched() task that is
235     * taking the early exit from freeing a data structure that
236     * is still being referenced (out of order) by the task
237     * doing the srcu_read_unlock().
238     *
239     * Alternatively, the comparison with "2" on the early exit
240     * could be changed to "3", but this increases synchronize_srcu()
241     * latency for bulk loads. So the current code is preferred.
242     */
243
244    mutex_unlock(&sp->mutex);
245}
246
247/**
248 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
249 * @sp: srcu_struct with which to synchronize.
250 *
251 * Flip the completed counter, and wait for the old count to drain to zero.
252 * As with classic RCU, the updater must use some separate means of
253 * synchronizing concurrent updates. Can block; must be called from
254 * process context.
255 *
256 * Note that it is illegal to call synchronize_srcu() from the corresponding
257 * SRCU read-side critical section; doing so will result in deadlock.
258 * However, it is perfectly legal to call synchronize_srcu() on one
259 * srcu_struct from some other srcu_struct's read-side critical section.
260 */
261void synchronize_srcu(struct srcu_struct *sp)
262{
263    __synchronize_srcu(sp, synchronize_sched);
264}
265EXPORT_SYMBOL_GPL(synchronize_srcu);
266
267/**
268 * synchronize_srcu_expedited - like synchronize_srcu, but less patient
269 * @sp: srcu_struct with which to synchronize.
270 *
271 * Flip the completed counter, and wait for the old count to drain to zero.
272 * As with classic RCU, the updater must use some separate means of
273 * synchronizing concurrent updates. Can block; must be called from
274 * process context.
275 *
276 * Note that it is illegal to call synchronize_srcu_expedited()
277 * from the corresponding SRCU read-side critical section; doing so
278 * will result in deadlock. However, it is perfectly legal to call
279 * synchronize_srcu_expedited() on one srcu_struct from some other
280 * srcu_struct's read-side critical section.
281 */
282void synchronize_srcu_expedited(struct srcu_struct *sp)
283{
284    __synchronize_srcu(sp, synchronize_sched_expedited);
285}
286EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
287
288/**
289 * srcu_batches_completed - return batches completed.
290 * @sp: srcu_struct on which to report batch completion.
291 *
292 * Report the number of batches, correlated with, but not necessarily
293 * precisely the same as, the number of grace periods that have elapsed.
294 */
295
296long srcu_batches_completed(struct srcu_struct *sp)
297{
298    return sp->completed;
299}
300EXPORT_SYMBOL_GPL(srcu_batches_completed);
301

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