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1 /*
2 * RCU-based infrastructure for lightweight reader-writer locking
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, you can access it online at
16 * http://www.gnu.org/licenses/gpl-2.0.html.
17 *
18 * Copyright (c) 2015, Red Hat, Inc.
19 *
20 * Author: Oleg Nesterov <oleg@redhat.com>
21 */
22
23 #include <linux/rcu_sync.h>
24 #include <linux/sched.h>
25
26 #ifdef CONFIG_PROVE_RCU
27 #define __INIT_HELD(func) .held = func,
28 #else
29 #define __INIT_HELD(func)
30 #endif
31
32 static const struct {
33 void (*sync)(void);
34 void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
35 void (*wait)(void);
36 #ifdef CONFIG_PROVE_RCU
37 int (*held)(void);
38 #endif
39 } gp_ops[] = {
40 [RCU_SYNC] = {
41 .sync = synchronize_rcu,
42 .call = call_rcu,
43 .wait = rcu_barrier,
44 __INIT_HELD(rcu_read_lock_held)
45 },
46 [RCU_SCHED_SYNC] = {
47 .sync = synchronize_sched,
48 .call = call_rcu_sched,
49 .wait = rcu_barrier_sched,
50 __INIT_HELD(rcu_read_lock_sched_held)
51 },
52 [RCU_BH_SYNC] = {
53 .sync = synchronize_rcu_bh,
54 .call = call_rcu_bh,
55 .wait = rcu_barrier_bh,
56 __INIT_HELD(rcu_read_lock_bh_held)
57 },
58 };
59
60 enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
61 enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
62
63 #define rss_lock gp_wait.lock
64
65 #ifdef CONFIG_PROVE_RCU
66 void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
67 {
68 RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
69 "suspicious rcu_sync_is_idle() usage");
70 }
71
72 EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
73 #endif
74
75 /**
76 * rcu_sync_init() - Initialize an rcu_sync structure
77 * @rsp: Pointer to rcu_sync structure to be initialized
78 * @type: Flavor of RCU with which to synchronize rcu_sync structure
79 */
80 void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
81 {
82 memset(rsp, 0, sizeof(*rsp));
83 init_waitqueue_head(&rsp->gp_wait);
84 rsp->gp_type = type;
85 }
86
87 /**
88 * Must be called after rcu_sync_init() and before first use.
89 *
90 * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
91 * pairs turn into NO-OPs.
92 */
93 void rcu_sync_enter_start(struct rcu_sync *rsp)
94 {
95 rsp->gp_count++;
96 rsp->gp_state = GP_PASSED;
97 }
98
99 /**
100 * rcu_sync_enter() - Force readers onto slowpath
101 * @rsp: Pointer to rcu_sync structure to use for synchronization
102 *
103 * This function is used by updaters who need readers to make use of
104 * a slowpath during the update. After this function returns, all
105 * subsequent calls to rcu_sync_is_idle() will return false, which
106 * tells readers to stay off their fastpaths. A later call to
107 * rcu_sync_exit() re-enables reader slowpaths.
108 *
109 * When called in isolation, rcu_sync_enter() must wait for a grace
110 * period, however, closely spaced calls to rcu_sync_enter() can
111 * optimize away the grace-period wait via a state machine implemented
112 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
113 */
114 void rcu_sync_enter(struct rcu_sync *rsp)
115 {
116 bool need_wait, need_sync;
117
118 spin_lock_irq(&rsp->rss_lock);
119 need_wait = rsp->gp_count++;
120 need_sync = rsp->gp_state == GP_IDLE;
121 if (need_sync)
122 rsp->gp_state = GP_PENDING;
123 spin_unlock_irq(&rsp->rss_lock);
124
125 BUG_ON(need_wait && need_sync);
126
127 if (need_sync) {
128 gp_ops[rsp->gp_type].sync();
129 rsp->gp_state = GP_PASSED;
130 wake_up_all(&rsp->gp_wait);
131 } else if (need_wait) {
132 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
133 } else {
134 /*
135 * Possible when there's a pending CB from a rcu_sync_exit().
136 * Nobody has yet been allowed the 'fast' path and thus we can
137 * avoid doing any sync(). The callback will get 'dropped'.
138 */
139 BUG_ON(rsp->gp_state != GP_PASSED);
140 }
141 }
142
143 /**
144 * rcu_sync_func() - Callback function managing reader access to fastpath
145 * @rsp: Pointer to rcu_sync structure to use for synchronization
146 *
147 * This function is passed to one of the call_rcu() functions by
148 * rcu_sync_exit(), so that it is invoked after a grace period following the
149 * that invocation of rcu_sync_exit(). It takes action based on events that
150 * have taken place in the meantime, so that closely spaced rcu_sync_enter()
151 * and rcu_sync_exit() pairs need not wait for a grace period.
152 *
153 * If another rcu_sync_enter() is invoked before the grace period
154 * ended, reset state to allow the next rcu_sync_exit() to let the
155 * readers back onto their fastpaths (after a grace period). If both
156 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
157 * before the grace period ended, re-invoke call_rcu() on behalf of that
158 * rcu_sync_exit(). Otherwise, set all state back to idle so that readers
159 * can again use their fastpaths.
160 */
161 static void rcu_sync_func(struct rcu_head *rcu)
162 {
163 struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
164 unsigned long flags;
165
166 BUG_ON(rsp->gp_state != GP_PASSED);
167 BUG_ON(rsp->cb_state == CB_IDLE);
168
169 spin_lock_irqsave(&rsp->rss_lock, flags);
170 if (rsp->gp_count) {
171 /*
172 * A new rcu_sync_begin() has happened; drop the callback.
173 */
174 rsp->cb_state = CB_IDLE;
175 } else if (rsp->cb_state == CB_REPLAY) {
176 /*
177 * A new rcu_sync_exit() has happened; requeue the callback
178 * to catch a later GP.
179 */
180 rsp->cb_state = CB_PENDING;
181 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
182 } else {
183 /*
184 * We're at least a GP after rcu_sync_exit(); eveybody will now
185 * have observed the write side critical section. Let 'em rip!.
186 */
187 rsp->cb_state = CB_IDLE;
188 rsp->gp_state = GP_IDLE;
189 }
190 spin_unlock_irqrestore(&rsp->rss_lock, flags);
191 }
192
193 /**
194 * rcu_sync_exit() - Allow readers back onto fast patch after grace period
195 * @rsp: Pointer to rcu_sync structure to use for synchronization
196 *
197 * This function is used by updaters who have completed, and can therefore
198 * now allow readers to make use of their fastpaths after a grace period
199 * has elapsed. After this grace period has completed, all subsequent
200 * calls to rcu_sync_is_idle() will return true, which tells readers that
201 * they can once again use their fastpaths.
202 */
203 void rcu_sync_exit(struct rcu_sync *rsp)
204 {
205 spin_lock_irq(&rsp->rss_lock);
206 if (!--rsp->gp_count) {
207 if (rsp->cb_state == CB_IDLE) {
208 rsp->cb_state = CB_PENDING;
209 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
210 } else if (rsp->cb_state == CB_PENDING) {
211 rsp->cb_state = CB_REPLAY;
212 }
213 }
214 spin_unlock_irq(&rsp->rss_lock);
215 }
216
217 /**
218 * rcu_sync_dtor() - Clean up an rcu_sync structure
219 * @rsp: Pointer to rcu_sync structure to be cleaned up
220 */
221 void rcu_sync_dtor(struct rcu_sync *rsp)
222 {
223 int cb_state;
224
225 BUG_ON(rsp->gp_count);
226
227 spin_lock_irq(&rsp->rss_lock);
228 if (rsp->cb_state == CB_REPLAY)
229 rsp->cb_state = CB_PENDING;
230 cb_state = rsp->cb_state;
231 spin_unlock_irq(&rsp->rss_lock);
232
233 if (cb_state != CB_IDLE) {
234 gp_ops[rsp->gp_type].wait();
235 BUG_ON(rsp->cb_state != CB_IDLE);
236 }
237 }