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Linux/kernel/rcu/sync.c

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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 }
238 

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