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 #endif 72 73 /** 74 * rcu_sync_init() - Initialize an rcu_sync structure 75 * @rsp: Pointer to rcu_sync structure to be initialized 76 * @type: Flavor of RCU with which to synchronize rcu_sync structure 77 */ 78 void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type) 79 { 80 memset(rsp, 0, sizeof(*rsp)); 81 init_waitqueue_head(&rsp->gp_wait); 82 rsp->gp_type = type; 83 } 84 85 /** 86 * rcu_sync_enter() - Force readers onto slowpath 87 * @rsp: Pointer to rcu_sync structure to use for synchronization 88 * 89 * This function is used by updaters who need readers to make use of 90 * a slowpath during the update. After this function returns, all 91 * subsequent calls to rcu_sync_is_idle() will return false, which 92 * tells readers to stay off their fastpaths. A later call to 93 * rcu_sync_exit() re-enables reader slowpaths. 94 * 95 * When called in isolation, rcu_sync_enter() must wait for a grace 96 * period, however, closely spaced calls to rcu_sync_enter() can 97 * optimize away the grace-period wait via a state machine implemented 98 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). 99 */ 100 void rcu_sync_enter(struct rcu_sync *rsp) 101 { 102 bool need_wait, need_sync; 103 104 spin_lock_irq(&rsp->rss_lock); 105 need_wait = rsp->gp_count++; 106 need_sync = rsp->gp_state == GP_IDLE; 107 if (need_sync) 108 rsp->gp_state = GP_PENDING; 109 spin_unlock_irq(&rsp->rss_lock); 110 111 BUG_ON(need_wait && need_sync); 112 113 if (need_sync) { 114 gp_ops[rsp->gp_type].sync(); 115 rsp->gp_state = GP_PASSED; 116 wake_up_all(&rsp->gp_wait); 117 } else if (need_wait) { 118 wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED); 119 } else { 120 /* 121 * Possible when there's a pending CB from a rcu_sync_exit(). 122 * Nobody has yet been allowed the 'fast' path and thus we can 123 * avoid doing any sync(). The callback will get 'dropped'. 124 */ 125 BUG_ON(rsp->gp_state != GP_PASSED); 126 } 127 } 128 129 /** 130 * rcu_sync_func() - Callback function managing reader access to fastpath 131 * @rsp: Pointer to rcu_sync structure to use for synchronization 132 * 133 * This function is passed to one of the call_rcu() functions by 134 * rcu_sync_exit(), so that it is invoked after a grace period following the 135 * that invocation of rcu_sync_exit(). It takes action based on events that 136 * have taken place in the meantime, so that closely spaced rcu_sync_enter() 137 * and rcu_sync_exit() pairs need not wait for a grace period. 138 * 139 * If another rcu_sync_enter() is invoked before the grace period 140 * ended, reset state to allow the next rcu_sync_exit() to let the 141 * readers back onto their fastpaths (after a grace period). If both 142 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked 143 * before the grace period ended, re-invoke call_rcu() on behalf of that 144 * rcu_sync_exit(). Otherwise, set all state back to idle so that readers 145 * can again use their fastpaths. 146 */ 147 static void rcu_sync_func(struct rcu_head *rcu) 148 { 149 struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head); 150 unsigned long flags; 151 152 BUG_ON(rsp->gp_state != GP_PASSED); 153 BUG_ON(rsp->cb_state == CB_IDLE); 154 155 spin_lock_irqsave(&rsp->rss_lock, flags); 156 if (rsp->gp_count) { 157 /* 158 * A new rcu_sync_begin() has happened; drop the callback. 159 */ 160 rsp->cb_state = CB_IDLE; 161 } else if (rsp->cb_state == CB_REPLAY) { 162 /* 163 * A new rcu_sync_exit() has happened; requeue the callback 164 * to catch a later GP. 165 */ 166 rsp->cb_state = CB_PENDING; 167 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); 168 } else { 169 /* 170 * We're at least a GP after rcu_sync_exit(); eveybody will now 171 * have observed the write side critical section. Let 'em rip!. 172 */ 173 rsp->cb_state = CB_IDLE; 174 rsp->gp_state = GP_IDLE; 175 } 176 spin_unlock_irqrestore(&rsp->rss_lock, flags); 177 } 178 179 /** 180 * rcu_sync_exit() - Allow readers back onto fast patch after grace period 181 * @rsp: Pointer to rcu_sync structure to use for synchronization 182 * 183 * This function is used by updaters who have completed, and can therefore 184 * now allow readers to make use of their fastpaths after a grace period 185 * has elapsed. After this grace period has completed, all subsequent 186 * calls to rcu_sync_is_idle() will return true, which tells readers that 187 * they can once again use their fastpaths. 188 */ 189 void rcu_sync_exit(struct rcu_sync *rsp) 190 { 191 spin_lock_irq(&rsp->rss_lock); 192 if (!--rsp->gp_count) { 193 if (rsp->cb_state == CB_IDLE) { 194 rsp->cb_state = CB_PENDING; 195 gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func); 196 } else if (rsp->cb_state == CB_PENDING) { 197 rsp->cb_state = CB_REPLAY; 198 } 199 } 200 spin_unlock_irq(&rsp->rss_lock); 201 } 202 203 /** 204 * rcu_sync_dtor() - Clean up an rcu_sync structure 205 * @rsp: Pointer to rcu_sync structure to be cleaned up 206 */ 207 void rcu_sync_dtor(struct rcu_sync *rsp) 208 { 209 int cb_state; 210 211 BUG_ON(rsp->gp_count); 212 213 spin_lock_irq(&rsp->rss_lock); 214 if (rsp->cb_state == CB_REPLAY) 215 rsp->cb_state = CB_PENDING; 216 cb_state = rsp->cb_state; 217 spin_unlock_irq(&rsp->rss_lock); 218 219 if (cb_state != CB_IDLE) { 220 gp_ops[rsp->gp_type].wait(); 221 BUG_ON(rsp->cb_state != CB_IDLE); 222 } 223 } 224