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cc44ca84 ON |
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 | ||
3a518b76 ON |
26 | #ifdef CONFIG_PROVE_RCU |
27 | #define __INIT_HELD(func) .held = func, | |
28 | #else | |
29 | #define __INIT_HELD(func) | |
30 | #endif | |
31 | ||
82e8c565 ON |
32 | static const struct { |
33 | void (*sync)(void); | |
34 | void (*call)(struct rcu_head *, void (*)(struct rcu_head *)); | |
07899a6e | 35 | void (*wait)(void); |
3a518b76 ON |
36 | #ifdef CONFIG_PROVE_RCU |
37 | int (*held)(void); | |
38 | #endif | |
82e8c565 ON |
39 | } gp_ops[] = { |
40 | [RCU_SYNC] = { | |
41 | .sync = synchronize_rcu, | |
42 | .call = call_rcu, | |
07899a6e | 43 | .wait = rcu_barrier, |
3a518b76 | 44 | __INIT_HELD(rcu_read_lock_held) |
82e8c565 ON |
45 | }, |
46 | [RCU_SCHED_SYNC] = { | |
47 | .sync = synchronize_sched, | |
48 | .call = call_rcu_sched, | |
07899a6e | 49 | .wait = rcu_barrier_sched, |
3a518b76 | 50 | __INIT_HELD(rcu_read_lock_sched_held) |
82e8c565 ON |
51 | }, |
52 | [RCU_BH_SYNC] = { | |
53 | .sync = synchronize_rcu_bh, | |
54 | .call = call_rcu_bh, | |
07899a6e | 55 | .wait = rcu_barrier_bh, |
3a518b76 | 56 | __INIT_HELD(rcu_read_lock_bh_held) |
82e8c565 ON |
57 | }, |
58 | }; | |
59 | ||
cc44ca84 ON |
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 | ||
3a518b76 | 65 | #ifdef CONFIG_PROVE_RCU |
4bace734 | 66 | void rcu_sync_lockdep_assert(struct rcu_sync *rsp) |
3a518b76 | 67 | { |
4bace734 ON |
68 | RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(), |
69 | "suspicious rcu_sync_is_idle() usage"); | |
3a518b76 ON |
70 | } |
71 | #endif | |
72 | ||
cc44ca84 ON |
73 | /** |
74 | * rcu_sync_init() - Initialize an rcu_sync structure | |
75 | * @rsp: Pointer to rcu_sync structure to be initialized | |
76 | * @type: Flavor of RCU with which to synchronize rcu_sync structure | |
77 | */ | |
78 | void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type) | |
79 | { | |
80 | memset(rsp, 0, sizeof(*rsp)); | |
81 | init_waitqueue_head(&rsp->gp_wait); | |
82e8c565 | 82 | rsp->gp_type = type; |
cc44ca84 ON |
83 | } |
84 | ||
85 | /** | |
86 | * rcu_sync_enter() - Force readers onto slowpath | |
87 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
88 | * | |
89 | * This function is used by updaters who need readers to make use of | |
90 | * a slowpath during the update. After this function returns, all | |
91 | * subsequent calls to rcu_sync_is_idle() will return false, which | |
92 | * tells readers to stay off their fastpaths. A later call to | |
93 | * rcu_sync_exit() re-enables reader slowpaths. | |
94 | * | |
95 | * When called in isolation, rcu_sync_enter() must wait for a grace | |
96 | * period, however, closely spaced calls to rcu_sync_enter() can | |
97 | * optimize away the grace-period wait via a state machine implemented | |
98 | * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). | |
99 | */ | |
100 | void rcu_sync_enter(struct rcu_sync *rsp) | |
101 | { | |
102 | bool need_wait, need_sync; | |
103 | ||
104 | spin_lock_irq(&rsp->rss_lock); | |
105 | need_wait = rsp->gp_count++; | |
106 | need_sync = rsp->gp_state == GP_IDLE; | |
107 | if (need_sync) | |
108 | rsp->gp_state = GP_PENDING; | |
109 | spin_unlock_irq(&rsp->rss_lock); | |
110 | ||
111 | BUG_ON(need_wait && need_sync); | |
112 | ||
113 | if (need_sync) { | |
82e8c565 | 114 | gp_ops[rsp->gp_type].sync(); |
cc44ca84 ON |
115 | rsp->gp_state = GP_PASSED; |
116 | wake_up_all(&rsp->gp_wait); | |
117 | } else if (need_wait) { | |
118 | wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED); | |
119 | } else { | |
120 | /* | |
121 | * Possible when there's a pending CB from a rcu_sync_exit(). | |
122 | * Nobody has yet been allowed the 'fast' path and thus we can | |
123 | * avoid doing any sync(). The callback will get 'dropped'. | |
124 | */ | |
125 | BUG_ON(rsp->gp_state != GP_PASSED); | |
126 | } | |
127 | } | |
128 | ||
129 | /** | |
130 | * rcu_sync_func() - Callback function managing reader access to fastpath | |
131 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
132 | * | |
133 | * This function is passed to one of the call_rcu() functions by | |
134 | * rcu_sync_exit(), so that it is invoked after a grace period following the | |
135 | * that invocation of rcu_sync_exit(). It takes action based on events that | |
136 | * have taken place in the meantime, so that closely spaced rcu_sync_enter() | |
137 | * and rcu_sync_exit() pairs need not wait for a grace period. | |
138 | * | |
139 | * If another rcu_sync_enter() is invoked before the grace period | |
140 | * ended, reset state to allow the next rcu_sync_exit() to let the | |
141 | * readers back onto their fastpaths (after a grace period). If both | |
142 | * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked | |
143 | * before the grace period ended, re-invoke call_rcu() on behalf of that | |
144 | * rcu_sync_exit(). Otherwise, set all state back to idle so that readers | |
145 | * can again use their fastpaths. | |
146 | */ | |
147 | static void rcu_sync_func(struct rcu_head *rcu) | |
148 | { | |
149 | struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head); | |
150 | unsigned long flags; | |
151 | ||
152 | BUG_ON(rsp->gp_state != GP_PASSED); | |
153 | BUG_ON(rsp->cb_state == CB_IDLE); | |
154 | ||
155 | spin_lock_irqsave(&rsp->rss_lock, flags); | |
156 | if (rsp->gp_count) { | |
157 | /* | |
158 | * A new rcu_sync_begin() has happened; drop the callback. | |
159 | */ | |
160 | rsp->cb_state = CB_IDLE; | |
161 | } else if (rsp->cb_state == CB_REPLAY) { | |
162 | /* | |
163 | * A new rcu_sync_exit() has happened; requeue the callback | |
164 | * to catch a later GP. | |
165 | */ | |
166 | rsp->cb_state = CB_PENDING; | |
82e8c565 | 167 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); |
cc44ca84 ON |
168 | } else { |
169 | /* | |
170 | * We're at least a GP after rcu_sync_exit(); eveybody will now | |
171 | * have observed the write side critical section. Let 'em rip!. | |
172 | */ | |
173 | rsp->cb_state = CB_IDLE; | |
174 | rsp->gp_state = GP_IDLE; | |
175 | } | |
176 | spin_unlock_irqrestore(&rsp->rss_lock, flags); | |
177 | } | |
178 | ||
179 | /** | |
180 | * rcu_sync_exit() - Allow readers back onto fast patch after grace period | |
181 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
182 | * | |
183 | * This function is used by updaters who have completed, and can therefore | |
184 | * now allow readers to make use of their fastpaths after a grace period | |
185 | * has elapsed. After this grace period has completed, all subsequent | |
186 | * calls to rcu_sync_is_idle() will return true, which tells readers that | |
187 | * they can once again use their fastpaths. | |
188 | */ | |
189 | void rcu_sync_exit(struct rcu_sync *rsp) | |
190 | { | |
191 | spin_lock_irq(&rsp->rss_lock); | |
192 | if (!--rsp->gp_count) { | |
193 | if (rsp->cb_state == CB_IDLE) { | |
194 | rsp->cb_state = CB_PENDING; | |
82e8c565 | 195 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); |
cc44ca84 ON |
196 | } else if (rsp->cb_state == CB_PENDING) { |
197 | rsp->cb_state = CB_REPLAY; | |
198 | } | |
199 | } | |
200 | spin_unlock_irq(&rsp->rss_lock); | |
201 | } | |
07899a6e ON |
202 | |
203 | /** | |
204 | * rcu_sync_dtor() - Clean up an rcu_sync structure | |
205 | * @rsp: Pointer to rcu_sync structure to be cleaned up | |
206 | */ | |
207 | void rcu_sync_dtor(struct rcu_sync *rsp) | |
208 | { | |
209 | int cb_state; | |
210 | ||
211 | BUG_ON(rsp->gp_count); | |
212 | ||
213 | spin_lock_irq(&rsp->rss_lock); | |
214 | if (rsp->cb_state == CB_REPLAY) | |
215 | rsp->cb_state = CB_PENDING; | |
216 | cb_state = rsp->cb_state; | |
217 | spin_unlock_irq(&rsp->rss_lock); | |
218 | ||
219 | if (cb_state != CB_IDLE) { | |
220 | gp_ops[rsp->gp_type].wait(); | |
221 | BUG_ON(rsp->cb_state != CB_IDLE); | |
222 | } | |
223 | } |