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 <[email protected]> 21 */ 22 23 #include <linux/rcu_sync.h> 24 #include <linux/sched.h> 25 26 enum { GP_IDLE = 0, GP_PENDING, GP_PASSED }; 27 enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY }; 28 29 #define rss_lock gp_wait.lock 30 31 /** 32 * rcu_sync_init() - Initialize an rcu_sync structure 33 * @rsp: Pointer to rcu_sync structure to be initialized 34 * @type: Flavor of RCU with which to synchronize rcu_sync structure 35 */ 36 void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type) 37 { 38 memset(rsp, 0, sizeof(*rsp)); 39 init_waitqueue_head(&rsp->gp_wait); 40 41 switch (type) { 42 case RCU_SYNC: 43 rsp->sync = synchronize_rcu; 44 rsp->call = call_rcu; 45 break; 46 47 case RCU_SCHED_SYNC: 48 rsp->sync = synchronize_sched; 49 rsp->call = call_rcu_sched; 50 break; 51 52 case RCU_BH_SYNC: 53 rsp->sync = synchronize_rcu_bh; 54 rsp->call = call_rcu_bh; 55 break; 56 } 57 } 58 59 /** 60 * rcu_sync_enter() - Force readers onto slowpath 61 * @rsp: Pointer to rcu_sync structure to use for synchronization 62 * 63 * This function is used by updaters who need readers to make use of 64 * a slowpath during the update. After this function returns, all 65 * subsequent calls to rcu_sync_is_idle() will return false, which 66 * tells readers to stay off their fastpaths. A later call to 67 * rcu_sync_exit() re-enables reader slowpaths. 68 * 69 * When called in isolation, rcu_sync_enter() must wait for a grace 70 * period, however, closely spaced calls to rcu_sync_enter() can 71 * optimize away the grace-period wait via a state machine implemented 72 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). 73 */ 74 void rcu_sync_enter(struct rcu_sync *rsp) 75 { 76 bool need_wait, need_sync; 77 78 spin_lock_irq(&rsp->rss_lock); 79 need_wait = rsp->gp_count++; 80 need_sync = rsp->gp_state == GP_IDLE; 81 if (need_sync) 82 rsp->gp_state = GP_PENDING; 83 spin_unlock_irq(&rsp->rss_lock); 84 85 BUG_ON(need_wait && need_sync); 86 87 if (need_sync) { 88 rsp->sync(); 89 rsp->gp_state = GP_PASSED; 90 wake_up_all(&rsp->gp_wait); 91 } else if (need_wait) { 92 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED); 93 } else { 94 /* 95 * Possible when there's a pending CB from a rcu_sync_exit(). 96 * Nobody has yet been allowed the 'fast' path and thus we can 97 * avoid doing any sync(). The callback will get 'dropped'. 98 */ 99 BUG_ON(rsp->gp_state != GP_PASSED); 100 } 101 } 102 103 /** 104 * rcu_sync_func() - Callback function managing reader access to fastpath 105 * @rsp: Pointer to rcu_sync structure to use for synchronization 106 * 107 * This function is passed to one of the call_rcu() functions by 108 * rcu_sync_exit(), so that it is invoked after a grace period following the 109 * that invocation of rcu_sync_exit(). It takes action based on events that 110 * have taken place in the meantime, so that closely spaced rcu_sync_enter() 111 * and rcu_sync_exit() pairs need not wait for a grace period. 112 * 113 * If another rcu_sync_enter() is invoked before the grace period 114 * ended, reset state to allow the next rcu_sync_exit() to let the 115 * readers back onto their fastpaths (after a grace period). If both 116 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked 117 * before the grace period ended, re-invoke call_rcu() on behalf of that 118 * rcu_sync_exit(). Otherwise, set all state back to idle so that readers 119 * can again use their fastpaths. 120 */ 121 static void rcu_sync_func(struct rcu_head *rcu) 122 { 123 struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head); 124 unsigned long flags; 125 126 BUG_ON(rsp->gp_state != GP_PASSED); 127 BUG_ON(rsp->cb_state == CB_IDLE); 128 129 spin_lock_irqsave(&rsp->rss_lock, flags); 130 if (rsp->gp_count) { 131 /* 132 * A new rcu_sync_begin() has happened; drop the callback. 133 */ 134 rsp->cb_state = CB_IDLE; 135 } else if (rsp->cb_state == CB_REPLAY) { 136 /* 137 * A new rcu_sync_exit() has happened; requeue the callback 138 * to catch a later GP. 139 */ 140 rsp->cb_state = CB_PENDING; 141 rsp->call(&rsp->cb_head, rcu_sync_func); 142 } else { 143 /* 144 * We're at least a GP after rcu_sync_exit(); eveybody will now 145 * have observed the write side critical section. Let 'em rip!. 146 */ 147 rsp->cb_state = CB_IDLE; 148 rsp->gp_state = GP_IDLE; 149 } 150 spin_unlock_irqrestore(&rsp->rss_lock, flags); 151 } 152 153 /** 154 * rcu_sync_exit() - Allow readers back onto fast patch after grace period 155 * @rsp: Pointer to rcu_sync structure to use for synchronization 156 * 157 * This function is used by updaters who have completed, and can therefore 158 * now allow readers to make use of their fastpaths after a grace period 159 * has elapsed. After this grace period has completed, all subsequent 160 * calls to rcu_sync_is_idle() will return true, which tells readers that 161 * they can once again use their fastpaths. 162 */ 163 void rcu_sync_exit(struct rcu_sync *rsp) 164 { 165 spin_lock_irq(&rsp->rss_lock); 166 if (!--rsp->gp_count) { 167 if (rsp->cb_state == CB_IDLE) { 168 rsp->cb_state = CB_PENDING; 169 rsp->call(&rsp->cb_head, rcu_sync_func); 170 } else if (rsp->cb_state == CB_PENDING) { 171 rsp->cb_state = CB_REPLAY; 172 } 173 } 174 spin_unlock_irq(&rsp->rss_lock); 175 } 176