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 } 241