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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 | int init_srcu_struct(struct srcu_struct *sp) |
46 | { |
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); |
51 | } |
52 | |
53 | /* |
54 | * srcu_readers_active_idx -- returns approximate number of readers |
55 | * active on the specified rank of per-CPU counters. |
56 | */ |
57 | |
58 | static int srcu_readers_active_idx(struct srcu_struct *sp, int idx) |
59 | { |
60 | int cpu; |
61 | int sum; |
62 | |
63 | sum = 0; |
64 | for_each_possible_cpu(cpu) |
65 | sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]; |
66 | return sum; |
67 | } |
68 | |
69 | /** |
70 | * srcu_readers_active - returns approximate number of readers. |
71 | * @sp: which srcu_struct to count active readers (holding srcu_read_lock). |
72 | * |
73 | * Note that this is not an atomic primitive, and can therefore suffer |
74 | * severe errors when invoked on an active srcu_struct. That said, it |
75 | * can be useful as an error check at cleanup time. |
76 | */ |
77 | static int srcu_readers_active(struct srcu_struct *sp) |
78 | { |
79 | return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1); |
80 | } |
81 | |
82 | /** |
83 | * cleanup_srcu_struct - deconstruct a sleep-RCU structure |
84 | * @sp: structure to clean up. |
85 | * |
86 | * Must invoke this after you are finished using a given srcu_struct that |
87 | * was initialized via init_srcu_struct(), else you leak memory. |
88 | */ |
89 | void cleanup_srcu_struct(struct srcu_struct *sp) |
90 | { |
91 | int sum; |
92 | |
93 | sum = srcu_readers_active(sp); |
94 | WARN_ON(sum); /* Leakage unless caller handles error. */ |
95 | if (sum != 0) |
96 | return; |
97 | free_percpu(sp->per_cpu_ref); |
98 | sp->per_cpu_ref = NULL; |
99 | } |
100 | |
101 | /** |
102 | * srcu_read_lock - register a new reader for an SRCU-protected structure. |
103 | * @sp: srcu_struct in which to register the new reader. |
104 | * |
105 | * Counts the new reader in the appropriate per-CPU element of the |
106 | * srcu_struct. Must be called from process context. |
107 | * Returns an index that must be passed to the matching srcu_read_unlock(). |
108 | */ |
109 | int srcu_read_lock(struct srcu_struct *sp) |
110 | { |
111 | int idx; |
112 | |
113 | preempt_disable(); |
114 | idx = sp->completed & 0x1; |
115 | barrier(); /* ensure compiler looks -once- at sp->completed. */ |
116 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; |
117 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ |
118 | preempt_enable(); |
119 | return idx; |
120 | } |
121 | |
122 | /** |
123 | * srcu_read_unlock - unregister a old reader from an SRCU-protected structure. |
124 | * @sp: srcu_struct in which to unregister the old reader. |
125 | * @idx: return value from corresponding srcu_read_lock(). |
126 | * |
127 | * Removes the count for the old reader from the appropriate per-CPU |
128 | * element of the srcu_struct. Note that this may well be a different |
129 | * CPU than that which was incremented by the corresponding srcu_read_lock(). |
130 | * Must be called from process context. |
131 | */ |
132 | void srcu_read_unlock(struct srcu_struct *sp, int idx) |
133 | { |
134 | preempt_disable(); |
135 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ |
136 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--; |
137 | preempt_enable(); |
138 | } |
139 | |
140 | /** |
141 | * synchronize_srcu - wait for prior SRCU read-side critical-section completion |
142 | * @sp: srcu_struct with which to synchronize. |
143 | * |
144 | * Flip the completed counter, and wait for the old count to drain to zero. |
145 | * As with classic RCU, the updater must use some separate means of |
146 | * synchronizing concurrent updates. Can block; must be called from |
147 | * process context. |
148 | * |
149 | * Note that it is illegal to call synchornize_srcu() from the corresponding |
150 | * SRCU read-side critical section; doing so will result in deadlock. |
151 | * However, it is perfectly legal to call synchronize_srcu() on one |
152 | * srcu_struct from some other srcu_struct's read-side critical section. |
153 | */ |
154 | void synchronize_srcu(struct srcu_struct *sp) |
155 | { |
156 | int idx; |
157 | |
158 | idx = sp->completed; |
159 | mutex_lock(&sp->mutex); |
160 | |
161 | /* |
162 | * Check to see if someone else did the work for us while we were |
163 | * waiting to acquire the lock. We need -two- advances of |
164 | * the counter, not just one. If there was but one, we might have |
165 | * shown up -after- our helper's first synchronize_sched(), thus |
166 | * having failed to prevent CPU-reordering races with concurrent |
167 | * srcu_read_unlock()s on other CPUs (see comment below). So we |
168 | * either (1) wait for two or (2) supply the second ourselves. |
169 | */ |
170 | |
171 | if ((sp->completed - idx) >= 2) { |
172 | mutex_unlock(&sp->mutex); |
173 | return; |
174 | } |
175 | |
176 | synchronize_sched(); /* Force memory barrier on all CPUs. */ |
177 | |
178 | /* |
179 | * The preceding synchronize_sched() ensures that any CPU that |
180 | * sees the new value of sp->completed will also see any preceding |
181 | * changes to data structures made by this CPU. This prevents |
182 | * some other CPU from reordering the accesses in its SRCU |
183 | * read-side critical section to precede the corresponding |
184 | * srcu_read_lock() -- ensuring that such references will in |
185 | * fact be protected. |
186 | * |
187 | * So it is now safe to do the flip. |
188 | */ |
189 | |
190 | idx = sp->completed & 0x1; |
191 | sp->completed++; |
192 | |
193 | synchronize_sched(); /* Force memory barrier on all CPUs. */ |
194 | |
195 | /* |
196 | * At this point, because of the preceding synchronize_sched(), |
197 | * all srcu_read_lock() calls using the old counters have completed. |
198 | * Their corresponding critical sections might well be still |
199 | * executing, but the srcu_read_lock() primitives themselves |
200 | * will have finished executing. |
201 | */ |
202 | |
203 | while (srcu_readers_active_idx(sp, idx)) |
204 | schedule_timeout_interruptible(1); |
205 | |
206 | synchronize_sched(); /* Force memory barrier on all CPUs. */ |
207 | |
208 | /* |
209 | * The preceding synchronize_sched() forces all srcu_read_unlock() |
210 | * primitives that were executing concurrently with the preceding |
211 | * for_each_possible_cpu() loop to have completed by this point. |
212 | * More importantly, it also forces the corresponding SRCU read-side |
213 | * critical sections to have also completed, and the corresponding |
214 | * references to SRCU-protected data items to be dropped. |
215 | * |
216 | * Note: |
217 | * |
218 | * Despite what you might think at first glance, the |
219 | * preceding synchronize_sched() -must- be within the |
220 | * critical section ended by the following mutex_unlock(). |
221 | * Otherwise, a task taking the early exit can race |
222 | * with a srcu_read_unlock(), which might have executed |
223 | * just before the preceding srcu_readers_active() check, |
224 | * and whose CPU might have reordered the srcu_read_unlock() |
225 | * with the preceding critical section. In this case, there |
226 | * is nothing preventing the synchronize_sched() task that is |
227 | * taking the early exit from freeing a data structure that |
228 | * is still being referenced (out of order) by the task |
229 | * doing the srcu_read_unlock(). |
230 | * |
231 | * Alternatively, the comparison with "2" on the early exit |
232 | * could be changed to "3", but this increases synchronize_srcu() |
233 | * latency for bulk loads. So the current code is preferred. |
234 | */ |
235 | |
236 | mutex_unlock(&sp->mutex); |
237 | } |
238 | |
239 | /** |
240 | * srcu_batches_completed - return batches completed. |
241 | * @sp: srcu_struct on which to report batch completion. |
242 | * |
243 | * Report the number of batches, correlated with, but not necessarily |
244 | * precisely the same as, the number of grace periods that have elapsed. |
245 | */ |
246 | |
247 | long srcu_batches_completed(struct srcu_struct *sp) |
248 | { |
249 | return sp->completed; |
250 | } |
251 | |
252 | EXPORT_SYMBOL_GPL(init_srcu_struct); |
253 | EXPORT_SYMBOL_GPL(cleanup_srcu_struct); |
254 | EXPORT_SYMBOL_GPL(srcu_read_lock); |
255 | EXPORT_SYMBOL_GPL(srcu_read_unlock); |
256 | EXPORT_SYMBOL_GPL(synchronize_srcu); |
257 | EXPORT_SYMBOL_GPL(srcu_batches_completed); |
258 |
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