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96b903f5 | 1 | // SPDX-License-Identifier: GPL-2.0+ |
cc44ca84 ON |
2 | /* |
3 | * RCU-based infrastructure for lightweight reader-writer locking | |
4 | * | |
cc44ca84 ON |
5 | * Copyright (c) 2015, Red Hat, Inc. |
6 | * | |
7 | * Author: Oleg Nesterov <oleg@redhat.com> | |
8 | */ | |
9 | ||
10 | #include <linux/rcu_sync.h> | |
11 | #include <linux/sched.h> | |
12 | ||
3a518b76 ON |
13 | #ifdef CONFIG_PROVE_RCU |
14 | #define __INIT_HELD(func) .held = func, | |
15 | #else | |
16 | #define __INIT_HELD(func) | |
17 | #endif | |
18 | ||
82e8c565 ON |
19 | static const struct { |
20 | void (*sync)(void); | |
21 | void (*call)(struct rcu_head *, void (*)(struct rcu_head *)); | |
07899a6e | 22 | void (*wait)(void); |
3a518b76 ON |
23 | #ifdef CONFIG_PROVE_RCU |
24 | int (*held)(void); | |
25 | #endif | |
82e8c565 ON |
26 | } gp_ops[] = { |
27 | [RCU_SYNC] = { | |
28 | .sync = synchronize_rcu, | |
29 | .call = call_rcu, | |
07899a6e | 30 | .wait = rcu_barrier, |
3a518b76 | 31 | __INIT_HELD(rcu_read_lock_held) |
82e8c565 ON |
32 | }, |
33 | [RCU_SCHED_SYNC] = { | |
d3ff3891 PM |
34 | .sync = synchronize_rcu, |
35 | .call = call_rcu, | |
36 | .wait = rcu_barrier, | |
3a518b76 | 37 | __INIT_HELD(rcu_read_lock_sched_held) |
82e8c565 ON |
38 | }, |
39 | [RCU_BH_SYNC] = { | |
d3ff3891 PM |
40 | .sync = synchronize_rcu, |
41 | .call = call_rcu, | |
42 | .wait = rcu_barrier, | |
3a518b76 | 43 | __INIT_HELD(rcu_read_lock_bh_held) |
82e8c565 ON |
44 | }, |
45 | }; | |
46 | ||
cc44ca84 ON |
47 | enum { GP_IDLE = 0, GP_PENDING, GP_PASSED }; |
48 | enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY }; | |
49 | ||
50 | #define rss_lock gp_wait.lock | |
51 | ||
3a518b76 | 52 | #ifdef CONFIG_PROVE_RCU |
4bace734 | 53 | void rcu_sync_lockdep_assert(struct rcu_sync *rsp) |
3a518b76 | 54 | { |
4bace734 ON |
55 | RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(), |
56 | "suspicious rcu_sync_is_idle() usage"); | |
3a518b76 | 57 | } |
80127a39 PZ |
58 | |
59 | EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert); | |
3a518b76 ON |
60 | #endif |
61 | ||
cc44ca84 ON |
62 | /** |
63 | * rcu_sync_init() - Initialize an rcu_sync structure | |
64 | * @rsp: Pointer to rcu_sync structure to be initialized | |
65 | * @type: Flavor of RCU with which to synchronize rcu_sync structure | |
66 | */ | |
67 | void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type) | |
68 | { | |
69 | memset(rsp, 0, sizeof(*rsp)); | |
70 | init_waitqueue_head(&rsp->gp_wait); | |
82e8c565 | 71 | rsp->gp_type = type; |
cc44ca84 ON |
72 | } |
73 | ||
3942a9bd | 74 | /** |
27fdb35f PM |
75 | * rcu_sync_enter_start - Force readers onto slow path for multiple updates |
76 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
77 | * | |
3942a9bd PZ |
78 | * Must be called after rcu_sync_init() and before first use. |
79 | * | |
80 | * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() | |
81 | * pairs turn into NO-OPs. | |
82 | */ | |
83 | void rcu_sync_enter_start(struct rcu_sync *rsp) | |
84 | { | |
85 | rsp->gp_count++; | |
86 | rsp->gp_state = GP_PASSED; | |
87 | } | |
88 | ||
cc44ca84 ON |
89 | /** |
90 | * rcu_sync_enter() - Force readers onto slowpath | |
91 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
92 | * | |
93 | * This function is used by updaters who need readers to make use of | |
94 | * a slowpath during the update. After this function returns, all | |
95 | * subsequent calls to rcu_sync_is_idle() will return false, which | |
96 | * tells readers to stay off their fastpaths. A later call to | |
97 | * rcu_sync_exit() re-enables reader slowpaths. | |
98 | * | |
99 | * When called in isolation, rcu_sync_enter() must wait for a grace | |
100 | * period, however, closely spaced calls to rcu_sync_enter() can | |
101 | * optimize away the grace-period wait via a state machine implemented | |
102 | * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). | |
103 | */ | |
104 | void rcu_sync_enter(struct rcu_sync *rsp) | |
105 | { | |
106 | bool need_wait, need_sync; | |
107 | ||
108 | spin_lock_irq(&rsp->rss_lock); | |
109 | need_wait = rsp->gp_count++; | |
110 | need_sync = rsp->gp_state == GP_IDLE; | |
111 | if (need_sync) | |
112 | rsp->gp_state = GP_PENDING; | |
113 | spin_unlock_irq(&rsp->rss_lock); | |
114 | ||
042d4c70 | 115 | WARN_ON_ONCE(need_wait && need_sync); |
cc44ca84 | 116 | if (need_sync) { |
82e8c565 | 117 | gp_ops[rsp->gp_type].sync(); |
cc44ca84 ON |
118 | rsp->gp_state = GP_PASSED; |
119 | wake_up_all(&rsp->gp_wait); | |
120 | } else if (need_wait) { | |
121 | wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED); | |
122 | } else { | |
123 | /* | |
124 | * Possible when there's a pending CB from a rcu_sync_exit(). | |
125 | * Nobody has yet been allowed the 'fast' path and thus we can | |
126 | * avoid doing any sync(). The callback will get 'dropped'. | |
127 | */ | |
042d4c70 | 128 | WARN_ON_ONCE(rsp->gp_state != GP_PASSED); |
cc44ca84 ON |
129 | } |
130 | } | |
131 | ||
132 | /** | |
133 | * rcu_sync_func() - Callback function managing reader access to fastpath | |
27fdb35f | 134 | * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization |
cc44ca84 ON |
135 | * |
136 | * This function is passed to one of the call_rcu() functions by | |
137 | * rcu_sync_exit(), so that it is invoked after a grace period following the | |
138 | * that invocation of rcu_sync_exit(). It takes action based on events that | |
139 | * have taken place in the meantime, so that closely spaced rcu_sync_enter() | |
140 | * and rcu_sync_exit() pairs need not wait for a grace period. | |
141 | * | |
142 | * If another rcu_sync_enter() is invoked before the grace period | |
143 | * ended, reset state to allow the next rcu_sync_exit() to let the | |
144 | * readers back onto their fastpaths (after a grace period). If both | |
145 | * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked | |
146 | * before the grace period ended, re-invoke call_rcu() on behalf of that | |
147 | * rcu_sync_exit(). Otherwise, set all state back to idle so that readers | |
148 | * can again use their fastpaths. | |
149 | */ | |
27fdb35f | 150 | static void rcu_sync_func(struct rcu_head *rhp) |
cc44ca84 | 151 | { |
27fdb35f | 152 | struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head); |
cc44ca84 ON |
153 | unsigned long flags; |
154 | ||
042d4c70 PM |
155 | WARN_ON_ONCE(rsp->gp_state != GP_PASSED); |
156 | WARN_ON_ONCE(rsp->cb_state == CB_IDLE); | |
cc44ca84 ON |
157 | |
158 | spin_lock_irqsave(&rsp->rss_lock, flags); | |
159 | if (rsp->gp_count) { | |
160 | /* | |
161 | * A new rcu_sync_begin() has happened; drop the callback. | |
162 | */ | |
163 | rsp->cb_state = CB_IDLE; | |
164 | } else if (rsp->cb_state == CB_REPLAY) { | |
165 | /* | |
166 | * A new rcu_sync_exit() has happened; requeue the callback | |
167 | * to catch a later GP. | |
168 | */ | |
169 | rsp->cb_state = CB_PENDING; | |
82e8c565 | 170 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); |
cc44ca84 ON |
171 | } else { |
172 | /* | |
173 | * We're at least a GP after rcu_sync_exit(); eveybody will now | |
174 | * have observed the write side critical section. Let 'em rip!. | |
175 | */ | |
176 | rsp->cb_state = CB_IDLE; | |
177 | rsp->gp_state = GP_IDLE; | |
178 | } | |
179 | spin_unlock_irqrestore(&rsp->rss_lock, flags); | |
180 | } | |
181 | ||
182 | /** | |
183 | * rcu_sync_exit() - Allow readers back onto fast patch after grace period | |
184 | * @rsp: Pointer to rcu_sync structure to use for synchronization | |
185 | * | |
186 | * This function is used by updaters who have completed, and can therefore | |
187 | * now allow readers to make use of their fastpaths after a grace period | |
188 | * has elapsed. After this grace period has completed, all subsequent | |
189 | * calls to rcu_sync_is_idle() will return true, which tells readers that | |
190 | * they can once again use their fastpaths. | |
191 | */ | |
192 | void rcu_sync_exit(struct rcu_sync *rsp) | |
193 | { | |
194 | spin_lock_irq(&rsp->rss_lock); | |
195 | if (!--rsp->gp_count) { | |
196 | if (rsp->cb_state == CB_IDLE) { | |
197 | rsp->cb_state = CB_PENDING; | |
82e8c565 | 198 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); |
cc44ca84 ON |
199 | } else if (rsp->cb_state == CB_PENDING) { |
200 | rsp->cb_state = CB_REPLAY; | |
201 | } | |
202 | } | |
203 | spin_unlock_irq(&rsp->rss_lock); | |
204 | } | |
07899a6e ON |
205 | |
206 | /** | |
207 | * rcu_sync_dtor() - Clean up an rcu_sync structure | |
208 | * @rsp: Pointer to rcu_sync structure to be cleaned up | |
209 | */ | |
210 | void rcu_sync_dtor(struct rcu_sync *rsp) | |
211 | { | |
212 | int cb_state; | |
213 | ||
042d4c70 | 214 | WARN_ON_ONCE(rsp->gp_count); |
07899a6e ON |
215 | |
216 | spin_lock_irq(&rsp->rss_lock); | |
217 | if (rsp->cb_state == CB_REPLAY) | |
218 | rsp->cb_state = CB_PENDING; | |
219 | cb_state = rsp->cb_state; | |
220 | spin_unlock_irq(&rsp->rss_lock); | |
221 | ||
222 | if (cb_state != CB_IDLE) { | |
223 | gp_ops[rsp->gp_type].wait(); | |
042d4c70 | 224 | WARN_ON_ONCE(rsp->cb_state != CB_IDLE); |
07899a6e ON |
225 | } |
226 | } |