kernel/rcu/sync.c

   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  * rcu_sync_enter_start - Force readers onto slow path for multiple updates
  89  * @rsp: Pointer to rcu_sync structure to use for synchronization
  90  *
  91  * Must be called after rcu_sync_init() and before first use.
  92  *
  93  * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
  94  * pairs turn into NO-OPs.
  95  */
  96 void rcu_sync_enter_start(struct rcu_sync *rsp)
  97 {
  98         rsp->gp_count++;
  99         rsp->gp_state = GP_PASSED;
 100 }
 101
 102 /**
 103  * rcu_sync_enter() - Force readers onto slowpath
 104  * @rsp: Pointer to rcu_sync structure to use for synchronization
 105  *
 106  * This function is used by updaters who need readers to make use of
 107  * a slowpath during the update.  After this function returns, all
 108  * subsequent calls to rcu_sync_is_idle() will return false, which
 109  * tells readers to stay off their fastpaths.  A later call to
 110  * rcu_sync_exit() re-enables reader slowpaths.
 111  *
 112  * When called in isolation, rcu_sync_enter() must wait for a grace
 113  * period, however, closely spaced calls to rcu_sync_enter() can
 114  * optimize away the grace-period wait via a state machine implemented
 115  * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
 116  */
 117 void rcu_sync_enter(struct rcu_sync *rsp)
 118 {
 119         bool need_wait, need_sync;
 120
 121         spin_lock_irq(&rsp->rss_lock);
 122         need_wait = rsp->gp_count++;
 123         need_sync = rsp->gp_state == GP_IDLE;
 124         if (need_sync)
 125                 rsp->gp_state = GP_PENDING;
 126         spin_unlock_irq(&rsp->rss_lock);
 127
 128         BUG_ON(need_wait && need_sync);
 129
 130         if (need_sync) {
 131                 gp_ops[rsp->gp_type].sync();
 132                 rsp->gp_state = GP_PASSED;
 133                 wake_up_all(&rsp->gp_wait);
 134         } else if (need_wait) {
 135                 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
 136         } else {
 137                 /*
 138                  * Possible when there's a pending CB from a rcu_sync_exit().
 139                  * Nobody has yet been allowed the 'fast' path and thus we can
 140                  * avoid doing any sync(). The callback will get 'dropped'.
 141                  */
 142                 BUG_ON(rsp->gp_state != GP_PASSED);
 143         }
 144 }
 145
 146 /**
 147  * rcu_sync_func() - Callback function managing reader access to fastpath
 148  * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
 149  *
 150  * This function is passed to one of the call_rcu() functions by
 151  * rcu_sync_exit(), so that it is invoked after a grace period following the
 152  * that invocation of rcu_sync_exit().  It takes action based on events that
 153  * have taken place in the meantime, so that closely spaced rcu_sync_enter()
 154  * and rcu_sync_exit() pairs need not wait for a grace period.
 155  *
 156  * If another rcu_sync_enter() is invoked before the grace period
 157  * ended, reset state to allow the next rcu_sync_exit() to let the
 158  * readers back onto their fastpaths (after a grace period).  If both
 159  * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
 160  * before the grace period ended, re-invoke call_rcu() on behalf of that
 161  * rcu_sync_exit().  Otherwise, set all state back to idle so that readers
 162  * can again use their fastpaths.
 163  */
 164 static void rcu_sync_func(struct rcu_head *rhp)
 165 {
 166         struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
 167         unsigned long flags;
 168
 169         BUG_ON(rsp->gp_state != GP_PASSED);
 170         BUG_ON(rsp->cb_state == CB_IDLE);
 171
 172         spin_lock_irqsave(&rsp->rss_lock, flags);
 173         if (rsp->gp_count) {
 174                 /*
 175                  * A new rcu_sync_begin() has happened; drop the callback.
 176                  */
 177                 rsp->cb_state = CB_IDLE;
 178         } else if (rsp->cb_state == CB_REPLAY) {
 179                 /*
 180                  * A new rcu_sync_exit() has happened; requeue the callback
 181                  * to catch a later GP.
 182                  */
 183                 rsp->cb_state = CB_PENDING;
 184                 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 185         } else {
 186                 /*
 187                  * We're at least a GP after rcu_sync_exit(); eveybody will now
 188                  * have observed the write side critical section. Let 'em rip!.
 189                  */
 190                 rsp->cb_state = CB_IDLE;
 191                 rsp->gp_state = GP_IDLE;
 192         }
 193         spin_unlock_irqrestore(&rsp->rss_lock, flags);
 194 }
 195
 196 /**
 197  * rcu_sync_exit() - Allow readers back onto fast patch after grace period
 198  * @rsp: Pointer to rcu_sync structure to use for synchronization
 199  *
 200  * This function is used by updaters who have completed, and can therefore
 201  * now allow readers to make use of their fastpaths after a grace period
 202  * has elapsed.  After this grace period has completed, all subsequent
 203  * calls to rcu_sync_is_idle() will return true, which tells readers that
 204  * they can once again use their fastpaths.
 205  */
 206 void rcu_sync_exit(struct rcu_sync *rsp)
 207 {
 208         spin_lock_irq(&rsp->rss_lock);
 209         if (!--rsp->gp_count) {
 210                 if (rsp->cb_state == CB_IDLE) {
 211                         rsp->cb_state = CB_PENDING;
 212                         gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
 213                 } else if (rsp->cb_state == CB_PENDING) {
 214                         rsp->cb_state = CB_REPLAY;
 215                 }
 216         }
 217         spin_unlock_irq(&rsp->rss_lock);
 218 }
 219
 220 /**
 221  * rcu_sync_dtor() - Clean up an rcu_sync structure
 222  * @rsp: Pointer to rcu_sync structure to be cleaned up
 223  */
 224 void rcu_sync_dtor(struct rcu_sync *rsp)
 225 {
 226         int cb_state;
 227
 228         BUG_ON(rsp->gp_count);
 229
 230         spin_lock_irq(&rsp->rss_lock);
 231         if (rsp->cb_state == CB_REPLAY)
 232                 rsp->cb_state = CB_PENDING;
 233         cb_state = rsp->cb_state;
 234         spin_unlock_irq(&rsp->rss_lock);
 235
 236         if (cb_state != CB_IDLE) {
 237                 gp_ops[rsp->gp_type].wait();
 238                 BUG_ON(rsp->cb_state != CB_IDLE);
 239         }
 240 }