TOMOYO: Use RCU primitives for list operation
[linux-2.6/linux-acpi-2.6/ibm-acpi-2.6.git] / kernel / srcu.c
blob818d7d9aa03c5f73a765f91dc0e56852960a93c4
1 /*
2 * Sleepable Read-Copy Update mechanism for mutual exclusion.
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.
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.
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.
18 * Copyright (C) IBM Corporation, 2006
20 * Author: Paul McKenney <paulmck@us.ibm.com>
22 * For detailed explanation of Read-Copy Update mechanism see -
23 * Documentation/RCU/ *.txt
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/slab.h>
34 #include <linux/smp.h>
35 #include <linux/srcu.h>
37 /**
38 * init_srcu_struct - initialize a sleep-RCU structure
39 * @sp: structure to initialize.
41 * Must invoke this on a given srcu_struct before passing that srcu_struct
42 * to any other function. Each srcu_struct represents a separate domain
43 * of SRCU protection.
45 int init_srcu_struct(struct srcu_struct *sp)
47 sp->completed = 0;
48 mutex_init(&sp->mutex);
49 sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
50 return (sp->per_cpu_ref ? 0 : -ENOMEM);
52 EXPORT_SYMBOL_GPL(init_srcu_struct);
55 * srcu_readers_active_idx -- returns approximate number of readers
56 * active on the specified rank of per-CPU counters.
59 static int srcu_readers_active_idx(struct srcu_struct *sp, int idx)
61 int cpu;
62 int sum;
64 sum = 0;
65 for_each_possible_cpu(cpu)
66 sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx];
67 return sum;
70 /**
71 * srcu_readers_active - returns approximate number of readers.
72 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
74 * Note that this is not an atomic primitive, and can therefore suffer
75 * severe errors when invoked on an active srcu_struct. That said, it
76 * can be useful as an error check at cleanup time.
78 static int srcu_readers_active(struct srcu_struct *sp)
80 return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1);
83 /**
84 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
85 * @sp: structure to clean up.
87 * Must invoke this after you are finished using a given srcu_struct that
88 * was initialized via init_srcu_struct(), else you leak memory.
90 void cleanup_srcu_struct(struct srcu_struct *sp)
92 int sum;
94 sum = srcu_readers_active(sp);
95 WARN_ON(sum); /* Leakage unless caller handles error. */
96 if (sum != 0)
97 return;
98 free_percpu(sp->per_cpu_ref);
99 sp->per_cpu_ref = NULL;
101 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
104 * srcu_read_lock - register a new reader for an SRCU-protected structure.
105 * @sp: srcu_struct in which to register the new reader.
107 * Counts the new reader in the appropriate per-CPU element of the
108 * srcu_struct. Must be called from process context.
109 * Returns an index that must be passed to the matching srcu_read_unlock().
111 int srcu_read_lock(struct srcu_struct *sp)
113 int idx;
115 preempt_disable();
116 idx = sp->completed & 0x1;
117 barrier(); /* ensure compiler looks -once- at sp->completed. */
118 per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++;
119 srcu_barrier(); /* ensure compiler won't misorder critical section. */
120 preempt_enable();
121 return idx;
123 EXPORT_SYMBOL_GPL(srcu_read_lock);
126 * srcu_read_unlock - unregister a old reader from an SRCU-protected structure.
127 * @sp: srcu_struct in which to unregister the old reader.
128 * @idx: return value from corresponding srcu_read_lock().
130 * Removes the count for the old reader from the appropriate per-CPU
131 * element of the srcu_struct. Note that this may well be a different
132 * CPU than that which was incremented by the corresponding srcu_read_lock().
133 * Must be called from process context.
135 void srcu_read_unlock(struct srcu_struct *sp, int idx)
137 preempt_disable();
138 srcu_barrier(); /* ensure compiler won't misorder critical section. */
139 per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--;
140 preempt_enable();
142 EXPORT_SYMBOL_GPL(srcu_read_unlock);
145 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
147 void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void))
149 int idx;
151 idx = sp->completed;
152 mutex_lock(&sp->mutex);
155 * Check to see if someone else did the work for us while we were
156 * waiting to acquire the lock. We need -two- advances of
157 * the counter, not just one. If there was but one, we might have
158 * shown up -after- our helper's first synchronize_sched(), thus
159 * having failed to prevent CPU-reordering races with concurrent
160 * srcu_read_unlock()s on other CPUs (see comment below). So we
161 * either (1) wait for two or (2) supply the second ourselves.
164 if ((sp->completed - idx) >= 2) {
165 mutex_unlock(&sp->mutex);
166 return;
169 sync_func(); /* Force memory barrier on all CPUs. */
172 * The preceding synchronize_sched() ensures that any CPU that
173 * sees the new value of sp->completed will also see any preceding
174 * changes to data structures made by this CPU. This prevents
175 * some other CPU from reordering the accesses in its SRCU
176 * read-side critical section to precede the corresponding
177 * srcu_read_lock() -- ensuring that such references will in
178 * fact be protected.
180 * So it is now safe to do the flip.
183 idx = sp->completed & 0x1;
184 sp->completed++;
186 sync_func(); /* Force memory barrier on all CPUs. */
189 * At this point, because of the preceding synchronize_sched(),
190 * all srcu_read_lock() calls using the old counters have completed.
191 * Their corresponding critical sections might well be still
192 * executing, but the srcu_read_lock() primitives themselves
193 * will have finished executing.
196 while (srcu_readers_active_idx(sp, idx))
197 schedule_timeout_interruptible(1);
199 sync_func(); /* Force memory barrier on all CPUs. */
202 * The preceding synchronize_sched() forces all srcu_read_unlock()
203 * primitives that were executing concurrently with the preceding
204 * for_each_possible_cpu() loop to have completed by this point.
205 * More importantly, it also forces the corresponding SRCU read-side
206 * critical sections to have also completed, and the corresponding
207 * references to SRCU-protected data items to be dropped.
209 * Note:
211 * Despite what you might think at first glance, the
212 * preceding synchronize_sched() -must- be within the
213 * critical section ended by the following mutex_unlock().
214 * Otherwise, a task taking the early exit can race
215 * with a srcu_read_unlock(), which might have executed
216 * just before the preceding srcu_readers_active() check,
217 * and whose CPU might have reordered the srcu_read_unlock()
218 * with the preceding critical section. In this case, there
219 * is nothing preventing the synchronize_sched() task that is
220 * taking the early exit from freeing a data structure that
221 * is still being referenced (out of order) by the task
222 * doing the srcu_read_unlock().
224 * Alternatively, the comparison with "2" on the early exit
225 * could be changed to "3", but this increases synchronize_srcu()
226 * latency for bulk loads. So the current code is preferred.
229 mutex_unlock(&sp->mutex);
233 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
234 * @sp: srcu_struct with which to synchronize.
236 * Flip the completed counter, and wait for the old count to drain to zero.
237 * As with classic RCU, the updater must use some separate means of
238 * synchronizing concurrent updates. Can block; must be called from
239 * process context.
241 * Note that it is illegal to call synchronize_srcu() from the corresponding
242 * SRCU read-side critical section; doing so will result in deadlock.
243 * However, it is perfectly legal to call synchronize_srcu() on one
244 * srcu_struct from some other srcu_struct's read-side critical section.
246 void synchronize_srcu(struct srcu_struct *sp)
248 __synchronize_srcu(sp, synchronize_sched);
250 EXPORT_SYMBOL_GPL(synchronize_srcu);
253 * synchronize_srcu_expedited - like synchronize_srcu, but less patient
254 * @sp: srcu_struct with which to synchronize.
256 * Flip the completed counter, and wait for the old count to drain to zero.
257 * As with classic RCU, the updater must use some separate means of
258 * synchronizing concurrent updates. Can block; must be called from
259 * process context.
261 * Note that it is illegal to call synchronize_srcu_expedited()
262 * from the corresponding SRCU read-side critical section; doing so
263 * will result in deadlock. However, it is perfectly legal to call
264 * synchronize_srcu_expedited() on one srcu_struct from some other
265 * srcu_struct's read-side critical section.
267 void synchronize_srcu_expedited(struct srcu_struct *sp)
269 __synchronize_srcu(sp, synchronize_sched_expedited);
271 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
274 * srcu_batches_completed - return batches completed.
275 * @sp: srcu_struct on which to report batch completion.
277 * Report the number of batches, correlated with, but not necessarily
278 * precisely the same as, the number of grace periods that have elapsed.
281 long srcu_batches_completed(struct srcu_struct *sp)
283 return sp->completed;
285 EXPORT_SYMBOL_GPL(srcu_batches_completed);