2 * RCU-based infrastructure for lightweight reader-writer locking
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.
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.
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.
18 * Copyright (c) 2015, Red Hat, Inc.
20 * Author: Oleg Nesterov <oleg@redhat.com>
23 #include <linux/rcu_sync.h>
24 #include <linux/sched.h>
26 #ifdef CONFIG_PROVE_RCU
27 #define __INIT_HELD(func) .held = func,
29 #define __INIT_HELD(func)
34 void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
36 #ifdef CONFIG_PROVE_RCU
41 .sync = synchronize_rcu,
44 __INIT_HELD(rcu_read_lock_held)
47 .sync = synchronize_sched,
48 .call = call_rcu_sched,
49 .wait = rcu_barrier_sched,
50 __INIT_HELD(rcu_read_lock_sched_held)
53 .sync = synchronize_rcu_bh,
55 .wait = rcu_barrier_bh,
56 __INIT_HELD(rcu_read_lock_bh_held)
60 enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
61 enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
63 #define rss_lock gp_wait.lock
65 #ifdef CONFIG_PROVE_RCU
66 void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
68 RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
69 "suspicious rcu_sync_is_idle() usage");
72 EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
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
80 void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
82 memset(rsp, 0, sizeof(*rsp));
83 init_waitqueue_head(&rsp->gp_wait);
88 * rcu_sync_enter() - Force readers onto slowpath
89 * @rsp: Pointer to rcu_sync structure to use for synchronization
91 * This function is used by updaters who need readers to make use of
92 * a slowpath during the update. After this function returns, all
93 * subsequent calls to rcu_sync_is_idle() will return false, which
94 * tells readers to stay off their fastpaths. A later call to
95 * rcu_sync_exit() re-enables reader slowpaths.
97 * When called in isolation, rcu_sync_enter() must wait for a grace
98 * period, however, closely spaced calls to rcu_sync_enter() can
99 * optimize away the grace-period wait via a state machine implemented
100 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
102 void rcu_sync_enter(struct rcu_sync *rsp)
104 bool need_wait, need_sync;
106 spin_lock_irq(&rsp->rss_lock);
107 need_wait = rsp->gp_count++;
108 need_sync = rsp->gp_state == GP_IDLE;
110 rsp->gp_state = GP_PENDING;
111 spin_unlock_irq(&rsp->rss_lock);
113 BUG_ON(need_wait && need_sync);
116 gp_ops[rsp->gp_type].sync();
117 rsp->gp_state = GP_PASSED;
118 wake_up_all(&rsp->gp_wait);
119 } else if (need_wait) {
120 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
123 * Possible when there's a pending CB from a rcu_sync_exit().
124 * Nobody has yet been allowed the 'fast' path and thus we can
125 * avoid doing any sync(). The callback will get 'dropped'.
127 BUG_ON(rsp->gp_state != GP_PASSED);
132 * rcu_sync_func() - Callback function managing reader access to fastpath
133 * @rsp: Pointer to rcu_sync structure to use for synchronization
135 * This function is passed to one of the call_rcu() functions by
136 * rcu_sync_exit(), so that it is invoked after a grace period following the
137 * that invocation of rcu_sync_exit(). It takes action based on events that
138 * have taken place in the meantime, so that closely spaced rcu_sync_enter()
139 * and rcu_sync_exit() pairs need not wait for a grace period.
141 * If another rcu_sync_enter() is invoked before the grace period
142 * ended, reset state to allow the next rcu_sync_exit() to let the
143 * readers back onto their fastpaths (after a grace period). If both
144 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
145 * before the grace period ended, re-invoke call_rcu() on behalf of that
146 * rcu_sync_exit(). Otherwise, set all state back to idle so that readers
147 * can again use their fastpaths.
149 static void rcu_sync_func(struct rcu_head *rcu)
151 struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);
154 BUG_ON(rsp->gp_state != GP_PASSED);
155 BUG_ON(rsp->cb_state == CB_IDLE);
157 spin_lock_irqsave(&rsp->rss_lock, flags);
160 * A new rcu_sync_begin() has happened; drop the callback.
162 rsp->cb_state = CB_IDLE;
163 } else if (rsp->cb_state == CB_REPLAY) {
165 * A new rcu_sync_exit() has happened; requeue the callback
166 * to catch a later GP.
168 rsp->cb_state = CB_PENDING;
169 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
172 * We're at least a GP after rcu_sync_exit(); eveybody will now
173 * have observed the write side critical section. Let 'em rip!.
175 rsp->cb_state = CB_IDLE;
176 rsp->gp_state = GP_IDLE;
178 spin_unlock_irqrestore(&rsp->rss_lock, flags);
182 * rcu_sync_exit() - Allow readers back onto fast patch after grace period
183 * @rsp: Pointer to rcu_sync structure to use for synchronization
185 * This function is used by updaters who have completed, and can therefore
186 * now allow readers to make use of their fastpaths after a grace period
187 * has elapsed. After this grace period has completed, all subsequent
188 * calls to rcu_sync_is_idle() will return true, which tells readers that
189 * they can once again use their fastpaths.
191 void rcu_sync_exit(struct rcu_sync *rsp)
193 spin_lock_irq(&rsp->rss_lock);
194 if (!--rsp->gp_count) {
195 if (rsp->cb_state == CB_IDLE) {
196 rsp->cb_state = CB_PENDING;
197 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
198 } else if (rsp->cb_state == CB_PENDING) {
199 rsp->cb_state = CB_REPLAY;
202 spin_unlock_irq(&rsp->rss_lock);
206 * rcu_sync_dtor() - Clean up an rcu_sync structure
207 * @rsp: Pointer to rcu_sync structure to be cleaned up
209 void rcu_sync_dtor(struct rcu_sync *rsp)
213 BUG_ON(rsp->gp_count);
215 spin_lock_irq(&rsp->rss_lock);
216 if (rsp->cb_state == CB_REPLAY)
217 rsp->cb_state = CB_PENDING;
218 cb_state = rsp->cb_state;
219 spin_unlock_irq(&rsp->rss_lock);
221 if (cb_state != CB_IDLE) {
222 gp_ops[rsp->gp_type].wait();
223 BUG_ON(rsp->cb_state != CB_IDLE);