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