| 1 | |
| 2 | * RCU-based infrastructure for lightweight reader-writer locking
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| 3 | *
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| 4 | * This program is free software; you can redistribute it and/or modify
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| 5 | * it under the terms of the GNU General Public License as published by
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| 6 | * the Free Software Foundation; either version 2 of the License, or
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| 7 | * (at your option) any later version.
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| 8 | *
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| 9 | * This program is distributed in the hope that it will be useful,
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| 10 | * but WITHOUT ANY WARRANTY; without even the implied warranty of
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| 11 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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| 12 | * GNU General Public License for more details.
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| 13 | *
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| 14 | * You should have received a copy of the GNU General Public License
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| 15 | * along with this program; if not, you can access it online at
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| 16 | * http://www.gnu.org/licenses/gpl-2.0.html.
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| 17 | *
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| 18 | * Copyright (c) 2015, Red Hat, Inc.
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| 19 | *
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| 20 | * Author: Oleg Nesterov <[email protected]>
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| 21 |
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| 22 |
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| 23 | #include <linux/rcu_sync.h>
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| 24 | #include <linux/sched.h>
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| 25 |
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| 26 | #ifdef CONFIG_PROVE_RCU
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| 27 | #define __INIT_HELD(func) .held = func,
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| 28 | #else
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| 29 | #define __INIT_HELD(func)
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| 30 | #endif
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| 31 |
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| 32 | static const struct {
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| 33 | void (*sync)(void);
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| 34 | void (*call)(struct rcu_head *, void (*)(struct rcu_head *));
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| 35 | void (*wait)(void);
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| 36 | #ifdef CONFIG_PROVE_RCU
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| 37 | int (*held)(void);
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| 38 | #endif
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| 39 | } gp_ops[] = {
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| 40 | [RCU_SYNC] = {
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| 41 | .sync = synchronize_rcu,
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| 42 | .call = call_rcu,
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| 43 | .wait = rcu_barrier,
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| 44 | __INIT_HELD(rcu_read_lock_held)
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| 45 | },
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| 46 | [RCU_SCHED_SYNC] = {
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| 47 | .sync = synchronize_sched,
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| 48 | .call = call_rcu_sched,
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| 49 | .wait = rcu_barrier_sched,
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| 50 | __INIT_HELD(rcu_read_lock_sched_held)
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| 51 | },
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| 52 | [RCU_BH_SYNC] = {
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| 53 | .sync = synchronize_rcu_bh,
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| 54 | .call = call_rcu_bh,
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| 55 | .wait = rcu_barrier_bh,
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| 56 | __INIT_HELD(rcu_read_lock_bh_held)
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| 57 | },
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| 58 | };
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| 59 |
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| 60 | enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };
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| 61 | enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY };
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| 62 |
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| 63 | #define rss_lock gp_wait.lock
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| 64 |
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| 65 | #ifdef CONFIG_PROVE_RCU
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| 66 | void rcu_sync_lockdep_assert(struct rcu_sync *rsp)
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| 67 | {
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| 68 | RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
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| 69 | "suspicious rcu_sync_is_idle() usage");
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| 70 | }
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| 71 |
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| 72 | EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);
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| 73 | #endif
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| 74 |
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| 75 | |
| 76 | * rcu_sync_init() - Initialize an rcu_sync structure
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| 77 | * @rsp: Pointer to rcu_sync structure to be initialized
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| 78 | * @type: Flavor of RCU with which to synchronize rcu_sync structure
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| 79 |
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| 80 | void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type)
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| 81 | {
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| 82 | memset(rsp, 0, sizeof(*rsp));
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| 83 | init_waitqueue_head(&rsp->gp_wait);
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| 84 | rsp->gp_type = type;
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| 85 | }
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| 86 |
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| 87 | |
| 88 | * rcu_sync_enter_start - Force readers onto slow path for multiple updates
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| 89 | * @rsp: Pointer to rcu_sync structure to use for synchronization
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| 90 | *
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| 91 | * Must be called after rcu_sync_init() and before first use.
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| 92 | *
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| 93 | * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}()
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| 94 | * pairs turn into NO-OPs.
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| 95 |
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| 96 | void rcu_sync_enter_start(struct rcu_sync *rsp)
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| 97 | {
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| 98 | rsp->gp_count++;
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| 99 | rsp->gp_state = GP_PASSED;
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| 100 | }
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| 101 |
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| 102 | |
| 103 | * rcu_sync_enter() - Force readers onto slowpath
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| 104 | * @rsp: Pointer to rcu_sync structure to use for synchronization
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| 105 | *
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| 106 | * This function is used by updaters who need readers to make use of
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| 107 | * a slowpath during the update. After this function returns, all
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| 108 | * subsequent calls to rcu_sync_is_idle() will return false, which
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| 109 | * tells readers to stay off their fastpaths. A later call to
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| 110 | * rcu_sync_exit() re-enables reader slowpaths.
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| 111 | *
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| 112 | * When called in isolation, rcu_sync_enter() must wait for a grace
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| 113 | * period, however, closely spaced calls to rcu_sync_enter() can
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| 114 | * optimize away the grace-period wait via a state machine implemented
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| 115 | * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func().
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| 116 |
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| 117 | void rcu_sync_enter(struct rcu_sync *rsp)
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| 118 | {
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| 119 | bool need_wait, need_sync;
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| 120 |
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| 121 | spin_lock_irq(&rsp->rss_lock);
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| 122 | need_wait = rsp->gp_count++;
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| 123 | need_sync = rsp->gp_state == GP_IDLE;
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| 124 | if (need_sync)
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| 125 | rsp->gp_state = GP_PENDING;
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| 126 | spin_unlock_irq(&rsp->rss_lock);
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| 127 |
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| 128 | BUG_ON(need_wait && need_sync);
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| 129 |
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| 130 | if (need_sync) {
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| 131 | gp_ops[rsp->gp_type].sync();
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| 132 | rsp->gp_state = GP_PASSED;
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| 133 | wake_up_all(&rsp->gp_wait);
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| 134 | } else if (need_wait) {
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| 135 | wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);
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| 136 | } else {
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| 137 | |
| 138 | * Possible when there's a pending CB from a rcu_sync_exit().
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| 139 | * Nobody has yet been allowed the 'fast' path and thus we can
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| 140 | * avoid doing any sync(). The callback will get 'dropped'.
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| 141 |
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| 142 | BUG_ON(rsp->gp_state != GP_PASSED);
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| 143 | }
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| 144 | }
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| 145 |
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| 146 | |
| 147 | * rcu_sync_func() - Callback function managing reader access to fastpath
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| 148 | * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization
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| 149 | *
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| 150 | * This function is passed to one of the call_rcu() functions by
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| 151 | * rcu_sync_exit(), so that it is invoked after a grace period following the
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| 152 | * that invocation of rcu_sync_exit(). It takes action based on events that
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| 153 | * have taken place in the meantime, so that closely spaced rcu_sync_enter()
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| 154 | * and rcu_sync_exit() pairs need not wait for a grace period.
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| 155 | *
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| 156 | * If another rcu_sync_enter() is invoked before the grace period
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| 157 | * ended, reset state to allow the next rcu_sync_exit() to let the
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| 158 | * readers back onto their fastpaths (after a grace period). If both
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| 159 | * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked
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| 160 | * before the grace period ended, re-invoke call_rcu() on behalf of that
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| 161 | * rcu_sync_exit(). Otherwise, set all state back to idle so that readers
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| 162 | * can again use their fastpaths.
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| 163 |
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| 164 | static void rcu_sync_func(struct rcu_head *rhp)
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| 165 | {
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| 166 | struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head);
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| 167 | unsigned long flags;
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| 168 |
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| 169 | BUG_ON(rsp->gp_state != GP_PASSED);
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| 170 | BUG_ON(rsp->cb_state == CB_IDLE);
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| 171 |
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| 172 | spin_lock_irqsave(&rsp->rss_lock, flags);
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| 173 | if (rsp->gp_count) {
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| 174 | |
| 175 | * A new rcu_sync_begin() has happened; drop the callback.
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| 176 |
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| 177 | rsp->cb_state = CB_IDLE;
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| 178 | } else if (rsp->cb_state == CB_REPLAY) {
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| 179 | |
| 180 | * A new rcu_sync_exit() has happened; requeue the callback
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| 181 | * to catch a later GP.
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| 182 |
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| 183 | rsp->cb_state = CB_PENDING;
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| 184 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
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| 185 | } else {
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| 186 | |
| 187 | * We're at least a GP after rcu_sync_exit(); eveybody will now
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| 188 | * have observed the write side critical section. Let 'em rip!.
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| 189 |
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| 190 | rsp->cb_state = CB_IDLE;
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| 191 | rsp->gp_state = GP_IDLE;
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| 192 | }
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| 193 | spin_unlock_irqrestore(&rsp->rss_lock, flags);
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| 194 | }
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| 195 |
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| 196 | |
| 197 | * rcu_sync_exit() - Allow readers back onto fast patch after grace period
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| 198 | * @rsp: Pointer to rcu_sync structure to use for synchronization
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| 199 | *
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| 200 | * This function is used by updaters who have completed, and can therefore
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| 201 | * now allow readers to make use of their fastpaths after a grace period
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| 202 | * has elapsed. After this grace period has completed, all subsequent
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| 203 | * calls to rcu_sync_is_idle() will return true, which tells readers that
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| 204 | * they can once again use their fastpaths.
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| 205 |
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| 206 | void rcu_sync_exit(struct rcu_sync *rsp)
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| 207 | {
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| 208 | spin_lock_irq(&rsp->rss_lock);
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| 209 | if (!--rsp->gp_count) {
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| 210 | if (rsp->cb_state == CB_IDLE) {
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| 211 | rsp->cb_state = CB_PENDING;
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| 212 | gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);
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| 213 | } else if (rsp->cb_state == CB_PENDING) {
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| 214 | rsp->cb_state = CB_REPLAY;
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| 215 | }
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| 216 | }
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| 217 | spin_unlock_irq(&rsp->rss_lock);
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| 218 | }
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| 219 |
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| 220 | |
| 221 | * rcu_sync_dtor() - Clean up an rcu_sync structure
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| 222 | * @rsp: Pointer to rcu_sync structure to be cleaned up
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| 223 |
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| 224 | void rcu_sync_dtor(struct rcu_sync *rsp)
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| 225 | {
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| 226 | int cb_state;
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| 227 |
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| 228 | BUG_ON(rsp->gp_count);
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| 229 |
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| 230 | spin_lock_irq(&rsp->rss_lock);
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| 231 | if (rsp->cb_state == CB_REPLAY)
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| 232 | rsp->cb_state = CB_PENDING;
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| 233 | cb_state = rsp->cb_state;
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| 234 | spin_unlock_irq(&rsp->rss_lock);
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| 235 |
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| 236 | if (cb_state != CB_IDLE) {
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| 237 | gp_ops[rsp->gp_type].wait();
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| 238 | BUG_ON(rsp->cb_state != CB_IDLE);
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| 239 | }
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| 240 | }
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