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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/slab.h> | |
34 | #include <linux/smp.h> | |
35 | #include <linux/srcu.h> | |
36 | ||
37 | /** | |
38 | * init_srcu_struct - initialize a sleep-RCU structure | |
39 | * @sp: structure to initialize. | |
40 | * | |
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. | |
44 | */ | |
45 | void init_srcu_struct(struct srcu_struct *sp) | |
46 | { | |
47 | sp->completed = 0; | |
48 | sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); | |
49 | mutex_init(&sp->mutex); | |
50 | } | |
51 | ||
52 | /* | |
53 | * srcu_readers_active_idx -- returns approximate number of readers | |
54 | * active on the specified rank of per-CPU counters. | |
55 | */ | |
56 | ||
57 | static int srcu_readers_active_idx(struct srcu_struct *sp, int idx) | |
58 | { | |
59 | int cpu; | |
60 | int sum; | |
61 | ||
62 | sum = 0; | |
63 | for_each_possible_cpu(cpu) | |
64 | sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]; | |
65 | return sum; | |
66 | } | |
67 | ||
68 | /** | |
69 | * srcu_readers_active - returns approximate number of readers. | |
70 | * @sp: which srcu_struct to count active readers (holding srcu_read_lock). | |
71 | * | |
72 | * Note that this is not an atomic primitive, and can therefore suffer | |
73 | * severe errors when invoked on an active srcu_struct. That said, it | |
74 | * can be useful as an error check at cleanup time. | |
75 | */ | |
76 | int srcu_readers_active(struct srcu_struct *sp) | |
77 | { | |
78 | return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1); | |
79 | } | |
80 | ||
81 | /** | |
82 | * cleanup_srcu_struct - deconstruct a sleep-RCU structure | |
83 | * @sp: structure to clean up. | |
84 | * | |
85 | * Must invoke this after you are finished using a given srcu_struct that | |
86 | * was initialized via init_srcu_struct(), else you leak memory. | |
87 | */ | |
88 | void cleanup_srcu_struct(struct srcu_struct *sp) | |
89 | { | |
90 | int sum; | |
91 | ||
92 | sum = srcu_readers_active(sp); | |
93 | WARN_ON(sum); /* Leakage unless caller handles error. */ | |
94 | if (sum != 0) | |
95 | return; | |
96 | free_percpu(sp->per_cpu_ref); | |
97 | sp->per_cpu_ref = NULL; | |
98 | } | |
99 | ||
100 | /** | |
101 | * srcu_read_lock - register a new reader for an SRCU-protected structure. | |
102 | * @sp: srcu_struct in which to register the new reader. | |
103 | * | |
104 | * Counts the new reader in the appropriate per-CPU element of the | |
105 | * srcu_struct. Must be called from process context. | |
106 | * Returns an index that must be passed to the matching srcu_read_unlock(). | |
107 | */ | |
108 | int srcu_read_lock(struct srcu_struct *sp) | |
109 | { | |
110 | int idx; | |
111 | ||
112 | preempt_disable(); | |
113 | idx = sp->completed & 0x1; | |
114 | barrier(); /* ensure compiler looks -once- at sp->completed. */ | |
115 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; | |
116 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ | |
117 | preempt_enable(); | |
118 | return idx; | |
119 | } | |
120 | ||
121 | /** | |
122 | * srcu_read_unlock - unregister a old reader from an SRCU-protected structure. | |
123 | * @sp: srcu_struct in which to unregister the old reader. | |
124 | * @idx: return value from corresponding srcu_read_lock(). | |
125 | * | |
126 | * Removes the count for the old reader from the appropriate per-CPU | |
127 | * element of the srcu_struct. Note that this may well be a different | |
128 | * CPU than that which was incremented by the corresponding srcu_read_lock(). | |
129 | * Must be called from process context. | |
130 | */ | |
131 | void srcu_read_unlock(struct srcu_struct *sp, int idx) | |
132 | { | |
133 | preempt_disable(); | |
134 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ | |
135 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--; | |
136 | preempt_enable(); | |
137 | } | |
138 | ||
139 | /** | |
140 | * synchronize_srcu - wait for prior SRCU read-side critical-section completion | |
141 | * @sp: srcu_struct with which to synchronize. | |
142 | * | |
143 | * Flip the completed counter, and wait for the old count to drain to zero. | |
144 | * As with classic RCU, the updater must use some separate means of | |
145 | * synchronizing concurrent updates. Can block; must be called from | |
146 | * process context. | |
147 | * | |
148 | * Note that it is illegal to call synchornize_srcu() from the corresponding | |
149 | * SRCU read-side critical section; doing so will result in deadlock. | |
150 | * However, it is perfectly legal to call synchronize_srcu() on one | |
151 | * srcu_struct from some other srcu_struct's read-side critical section. | |
152 | */ | |
153 | void synchronize_srcu(struct srcu_struct *sp) | |
154 | { | |
155 | int idx; | |
156 | ||
157 | idx = sp->completed; | |
158 | mutex_lock(&sp->mutex); | |
159 | ||
160 | /* | |
161 | * Check to see if someone else did the work for us while we were | |
162 | * waiting to acquire the lock. We need -two- advances of | |
163 | * the counter, not just one. If there was but one, we might have | |
164 | * shown up -after- our helper's first synchronize_sched(), thus | |
165 | * having failed to prevent CPU-reordering races with concurrent | |
166 | * srcu_read_unlock()s on other CPUs (see comment below). So we | |
167 | * either (1) wait for two or (2) supply the second ourselves. | |
168 | */ | |
169 | ||
170 | if ((sp->completed - idx) >= 2) { | |
171 | mutex_unlock(&sp->mutex); | |
172 | return; | |
173 | } | |
174 | ||
175 | synchronize_sched(); /* Force memory barrier on all CPUs. */ | |
176 | ||
177 | /* | |
178 | * The preceding synchronize_sched() ensures that any CPU that | |
179 | * sees the new value of sp->completed will also see any preceding | |
180 | * changes to data structures made by this CPU. This prevents | |
181 | * some other CPU from reordering the accesses in its SRCU | |
182 | * read-side critical section to precede the corresponding | |
183 | * srcu_read_lock() -- ensuring that such references will in | |
184 | * fact be protected. | |
185 | * | |
186 | * So it is now safe to do the flip. | |
187 | */ | |
188 | ||
189 | idx = sp->completed & 0x1; | |
190 | sp->completed++; | |
191 | ||
192 | synchronize_sched(); /* Force memory barrier on all CPUs. */ | |
193 | ||
194 | /* | |
195 | * At this point, because of the preceding synchronize_sched(), | |
196 | * all srcu_read_lock() calls using the old counters have completed. | |
197 | * Their corresponding critical sections might well be still | |
198 | * executing, but the srcu_read_lock() primitives themselves | |
199 | * will have finished executing. | |
200 | */ | |
201 | ||
202 | while (srcu_readers_active_idx(sp, idx)) | |
203 | schedule_timeout_interruptible(1); | |
204 | ||
205 | synchronize_sched(); /* Force memory barrier on all CPUs. */ | |
206 | ||
207 | /* | |
208 | * The preceding synchronize_sched() forces all srcu_read_unlock() | |
209 | * primitives that were executing concurrently with the preceding | |
210 | * for_each_possible_cpu() loop to have completed by this point. | |
211 | * More importantly, it also forces the corresponding SRCU read-side | |
212 | * critical sections to have also completed, and the corresponding | |
213 | * references to SRCU-protected data items to be dropped. | |
214 | * | |
215 | * Note: | |
216 | * | |
217 | * Despite what you might think at first glance, the | |
218 | * preceding synchronize_sched() -must- be within the | |
219 | * critical section ended by the following mutex_unlock(). | |
220 | * Otherwise, a task taking the early exit can race | |
221 | * with a srcu_read_unlock(), which might have executed | |
222 | * just before the preceding srcu_readers_active() check, | |
223 | * and whose CPU might have reordered the srcu_read_unlock() | |
224 | * with the preceding critical section. In this case, there | |
225 | * is nothing preventing the synchronize_sched() task that is | |
226 | * taking the early exit from freeing a data structure that | |
227 | * is still being referenced (out of order) by the task | |
228 | * doing the srcu_read_unlock(). | |
229 | * | |
230 | * Alternatively, the comparison with "2" on the early exit | |
231 | * could be changed to "3", but this increases synchronize_srcu() | |
232 | * latency for bulk loads. So the current code is preferred. | |
233 | */ | |
234 | ||
235 | mutex_unlock(&sp->mutex); | |
236 | } | |
237 | ||
238 | /** | |
239 | * srcu_batches_completed - return batches completed. | |
240 | * @sp: srcu_struct on which to report batch completion. | |
241 | * | |
242 | * Report the number of batches, correlated with, but not necessarily | |
243 | * precisely the same as, the number of grace periods that have elapsed. | |
244 | */ | |
245 | ||
246 | long srcu_batches_completed(struct srcu_struct *sp) | |
247 | { | |
248 | return sp->completed; | |
249 | } | |
250 | ||
251 | EXPORT_SYMBOL_GPL(init_srcu_struct); | |
252 | EXPORT_SYMBOL_GPL(cleanup_srcu_struct); | |
253 | EXPORT_SYMBOL_GPL(srcu_read_lock); | |
254 | EXPORT_SYMBOL_GPL(srcu_read_unlock); | |
255 | EXPORT_SYMBOL_GPL(synchronize_srcu); | |
256 | EXPORT_SYMBOL_GPL(srcu_batches_completed); | |
257 | EXPORT_SYMBOL_GPL(srcu_readers_active); |