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/delay.h>
35#include <linux/srcu.h>
36
37static int init_srcu_struct_fields(struct srcu_struct *sp)
38{
39    sp->completed = 0;
40    mutex_init(&sp->mutex);
41    sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
42    return sp->per_cpu_ref ? 0 : -ENOMEM;
43}
44
45#ifdef CONFIG_DEBUG_LOCK_ALLOC
46
47int __init_srcu_struct(struct srcu_struct *sp, const char *name,
48               struct lock_class_key *key)
49{
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    return init_srcu_struct_fields(sp);
54}
55EXPORT_SYMBOL_GPL(__init_srcu_struct);
56
57#else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
58
59/**
60 * init_srcu_struct - initialize a sleep-RCU structure
61 * @sp: structure to initialize.
62 *
63 * Must invoke this on a given srcu_struct before passing that srcu_struct
64 * to any other function. Each srcu_struct represents a separate domain
65 * of SRCU protection.
66 */
67int init_srcu_struct(struct srcu_struct *sp)
68{
69    return init_srcu_struct_fields(sp);
70}
71EXPORT_SYMBOL_GPL(init_srcu_struct);
72
73#endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
74
75/*
76 * srcu_readers_active_idx -- returns approximate number of readers
77 * active on the specified rank of per-CPU counters.
78 */
79
80static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
81{
82    int cpu;
83    int sum;
84
85    sum = 0;
86    for_each_possible_cpu(cpu)
87        sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
88    return sum;
89}
90
91/**
92 * srcu_readers_active - returns approximate number of readers.
93 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
94 *
95 * Note that this is not an atomic primitive, and can therefore suffer
96 * severe errors when invoked on an active srcu_struct. That said, it
97 * can be useful as an error check at cleanup time.
98 */
99static int srcu_readers_active(struct srcu_struct *sp)
100{
101    return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
102}
103
104/**
105 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
106 * @sp: structure to clean up.
107 *
108 * Must invoke this after you are finished using a given srcu_struct that
109 * was initialized via init_srcu_struct(), else you leak memory.
110 */
111void cleanup_srcu_struct(struct srcu_struct *sp)
112{
113    int sum;
114
115    sum = srcu_readers_active(sp);
116    WARN_ON(sum); /* Leakage unless caller handles error. */
117    if (sum != 0)
118        return;
119    free_percpu(sp->per_cpu_ref);
120    sp->per_cpu_ref = NULL;
121}
122EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
123
124/*
125 * Counts the new reader in the appropriate per-CPU element of the
126 * srcu_struct. Must be called from process context.
127 * Returns an index that must be passed to the matching srcu_read_unlock().
128 */
129int __srcu_read_lock(struct srcu_struct *sp)
130{
131    int idx;
132
133    preempt_disable();
134    idx = sp->completed & 0x1;
135    barrier(); /* ensure compiler looks -once- at sp->completed. */
136    per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
137    srcu_barrier(); /* ensure compiler won't misorder critical section. */
138    preempt_enable();
139    return idx;
140}
141EXPORT_SYMBOL_GPL(__srcu_read_lock);
142
143/*
144 * Removes the count for the old reader from the appropriate per-CPU
145 * element of the srcu_struct. Note that this may well be a different
146 * CPU than that which was incremented by the corresponding srcu_read_lock().
147 * Must be called from process context.
148 */
149void __srcu_read_unlock(struct srcu_struct *sp, int idx)
150{
151    preempt_disable();
152    srcu_barrier(); /* ensure compiler won't misorder critical section. */
153    per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
154    preempt_enable();
155}
156EXPORT_SYMBOL_GPL(__srcu_read_unlock);
157
158/*
159 * We use an adaptive strategy for synchronize_srcu() and especially for
160 * synchronize_srcu_expedited(). We spin for a fixed time period
161 * (defined below) to allow SRCU readers to exit their read-side critical
162 * sections. If there are still some readers after 10 microseconds,
163 * we repeatedly block for 1-millisecond time periods. This approach
164 * has done well in testing, so there is no need for a config parameter.
165 */
166#define SYNCHRONIZE_SRCU_READER_DELAY 10
167
168/*
169 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
170 */
171static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
172{
173    int idx;
174
175    idx = sp->completed;
176    mutex_lock(&sp->mutex);
177
178    /*
179     * Check to see if someone else did the work for us while we were
180     * waiting to acquire the lock. We need -two- advances of
181     * the counter, not just one. If there was but one, we might have
182     * shown up -after- our helper's first synchronize_sched(), thus
183     * having failed to prevent CPU-reordering races with concurrent
184     * srcu_read_unlock()s on other CPUs (see comment below). So we
185     * either (1) wait for two or (2) supply the second ourselves.
186     */
187
188    if ((sp->completed - idx) >= 2) {
189        mutex_unlock(&sp->mutex);
190        return;
191    }
192
193    sync_func(); /* Force memory barrier on all CPUs. */
194
195    /*
196     * The preceding synchronize_sched() ensures that any CPU that
197     * sees the new value of sp->completed will also see any preceding
198     * changes to data structures made by this CPU. This prevents
199     * some other CPU from reordering the accesses in its SRCU
200     * read-side critical section to precede the corresponding
201     * srcu_read_lock() -- ensuring that such references will in
202     * fact be protected.
203     *
204     * So it is now safe to do the flip.
205     */
206
207    idx = sp->completed & 0x1;
208    sp->completed++;
209
210    sync_func(); /* Force memory barrier on all CPUs. */
211
212    /*
213     * At this point, because of the preceding synchronize_sched(),
214     * all srcu_read_lock() calls using the old counters have completed.
215     * Their corresponding critical sections might well be still
216     * executing, but the srcu_read_lock() primitives themselves
217     * will have finished executing. We initially give readers
218     * an arbitrarily chosen 10 microseconds to get out of their
219     * SRCU read-side critical sections, then loop waiting 1/HZ
220     * seconds per iteration. The 10-microsecond value has done
221     * very well in testing.
222     */
223
224    if (srcu_readers_active_idx(sp, idx))
225        udelay(SYNCHRONIZE_SRCU_READER_DELAY);
226    while (srcu_readers_active_idx(sp, idx))
227        schedule_timeout_interruptible(1);
228
229    sync_func(); /* Force memory barrier on all CPUs. */
230
231    /*
232     * The preceding synchronize_sched() forces all srcu_read_unlock()
233     * primitives that were executing concurrently with the preceding
234     * for_each_possible_cpu() loop to have completed by this point.
235     * More importantly, it also forces the corresponding SRCU read-side
236     * critical sections to have also completed, and the corresponding
237     * references to SRCU-protected data items to be dropped.
238     *
239     * Note:
240     *
241     * Despite what you might think at first glance, the
242     * preceding synchronize_sched() -must- be within the
243     * critical section ended by the following mutex_unlock().
244     * Otherwise, a task taking the early exit can race
245     * with a srcu_read_unlock(), which might have executed
246     * just before the preceding srcu_readers_active() check,
247     * and whose CPU might have reordered the srcu_read_unlock()
248     * with the preceding critical section. In this case, there
249     * is nothing preventing the synchronize_sched() task that is
250     * taking the early exit from freeing a data structure that
251     * is still being referenced (out of order) by the task
252     * doing the srcu_read_unlock().
253     *
254     * Alternatively, the comparison with "2" on the early exit
255     * could be changed to "3", but this increases synchronize_srcu()
256     * latency for bulk loads. So the current code is preferred.
257     */
258
259    mutex_unlock(&sp->mutex);
260}
261
262/**
263 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
264 * @sp: srcu_struct with which to synchronize.
265 *
266 * Flip the completed counter, and wait for the old count to drain to zero.
267 * As with classic RCU, the updater must use some separate means of
268 * synchronizing concurrent updates. Can block; must be called from
269 * process context.
270 *
271 * Note that it is illegal to call synchronize_srcu() from the corresponding
272 * SRCU read-side critical section; doing so will result in deadlock.
273 * However, it is perfectly legal to call synchronize_srcu() on one
274 * srcu_struct from some other srcu_struct's read-side critical section.
275 */
276void synchronize_srcu(struct srcu_struct *sp)
277{
278    __synchronize_srcu(sp, synchronize_sched);
279}
280EXPORT_SYMBOL_GPL(synchronize_srcu);
281
282/**
283 * synchronize_srcu_expedited - like synchronize_srcu, but less patient
284 * @sp: srcu_struct with which to synchronize.
285 *
286 * Flip the completed counter, and wait for the old count to drain to zero.
287 * As with classic RCU, the updater must use some separate means of
288 * synchronizing concurrent updates. Can block; must be called from
289 * process context.
290 *
291 * Note that it is illegal to call synchronize_srcu_expedited()
292 * from the corresponding SRCU read-side critical section; doing so
293 * will result in deadlock. However, it is perfectly legal to call
294 * synchronize_srcu_expedited() on one srcu_struct from some other
295 * srcu_struct's read-side critical section.
296 */
297void synchronize_srcu_expedited(struct srcu_struct *sp)
298{
299    __synchronize_srcu(sp, synchronize_sched_expedited);
300}
301EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
302
303/**
304 * srcu_batches_completed - return batches completed.
305 * @sp: srcu_struct on which to report batch completion.
306 *
307 * Report the number of batches, correlated with, but not necessarily
308 * precisely the same as, the number of grace periods that have elapsed.
309 */
310
311long srcu_batches_completed(struct srcu_struct *sp)
312{
313    return sp->completed;
314}
315EXPORT_SYMBOL_GPL(srcu_batches_completed);
316

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