1 // SPDX-License-Identifier: GPL-2.0+ 2 /* 3 * RCU-based infrastructure for lightweight reader-writer locking 4 * 5 * Copyright (c) 2015, Red Hat, Inc. 6 * 7 * Author: Oleg Nesterov <oleg@redhat.com> 8 */ 9 10 #include <linux/rcu_sync.h> 11 #include <linux/sched.h> 12 13 enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY }; 14 15 #define rss_lock gp_wait.lock 16 17 /** 18 * rcu_sync_init() - Initialize an rcu_sync structure 19 * @rsp: Pointer to rcu_sync structure to be initialized 20 */ 21 void rcu_sync_init(struct rcu_sync *rsp) 22 { 23 memset(rsp, 0, sizeof(*rsp)); 24 init_waitqueue_head(&rsp->gp_wait); 25 } 26 27 /** 28 * rcu_sync_enter_start - Force readers onto slow path for multiple updates 29 * @rsp: Pointer to rcu_sync structure to use for synchronization 30 * 31 * Must be called after rcu_sync_init() and before first use. 32 * 33 * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() 34 * pairs turn into NO-OPs. 35 */ 36 void rcu_sync_enter_start(struct rcu_sync *rsp) 37 { 38 rsp->gp_count++; 39 rsp->gp_state = GP_PASSED; 40 } 41 42 43 static void rcu_sync_func(struct rcu_head *rhp); 44 45 static void rcu_sync_call(struct rcu_sync *rsp) 46 { 47 call_rcu(&rsp->cb_head, rcu_sync_func); 48 } 49 50 /** 51 * rcu_sync_func() - Callback function managing reader access to fastpath 52 * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization 53 * 54 * This function is passed to call_rcu() function by rcu_sync_enter() and 55 * rcu_sync_exit(), so that it is invoked after a grace period following the 56 * that invocation of enter/exit. 57 * 58 * If it is called by rcu_sync_enter() it signals that all the readers were 59 * switched onto slow path. 60 * 61 * If it is called by rcu_sync_exit() it takes action based on events that 62 * have taken place in the meantime, so that closely spaced rcu_sync_enter() 63 * and rcu_sync_exit() pairs need not wait for a grace period. 64 * 65 * If another rcu_sync_enter() is invoked before the grace period 66 * ended, reset state to allow the next rcu_sync_exit() to let the 67 * readers back onto their fastpaths (after a grace period). If both 68 * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked 69 * before the grace period ended, re-invoke call_rcu() on behalf of that 70 * rcu_sync_exit(). Otherwise, set all state back to idle so that readers 71 * can again use their fastpaths. 72 */ 73 static void rcu_sync_func(struct rcu_head *rhp) 74 { 75 struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head); 76 unsigned long flags; 77 78 WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); 79 WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); 80 81 spin_lock_irqsave(&rsp->rss_lock, flags); 82 if (rsp->gp_count) { 83 /* 84 * We're at least a GP after the GP_IDLE->GP_ENTER transition. 85 */ 86 WRITE_ONCE(rsp->gp_state, GP_PASSED); 87 wake_up_locked(&rsp->gp_wait); 88 } else if (rsp->gp_state == GP_REPLAY) { 89 /* 90 * A new rcu_sync_exit() has happened; requeue the callback to 91 * catch a later GP. 92 */ 93 WRITE_ONCE(rsp->gp_state, GP_EXIT); 94 rcu_sync_call(rsp); 95 } else { 96 /* 97 * We're at least a GP after the last rcu_sync_exit(); eveybody 98 * will now have observed the write side critical section. 99 * Let 'em rip!. 100 */ 101 WRITE_ONCE(rsp->gp_state, GP_IDLE); 102 } 103 spin_unlock_irqrestore(&rsp->rss_lock, flags); 104 } 105 106 /** 107 * rcu_sync_enter() - Force readers onto slowpath 108 * @rsp: Pointer to rcu_sync structure to use for synchronization 109 * 110 * This function is used by updaters who need readers to make use of 111 * a slowpath during the update. After this function returns, all 112 * subsequent calls to rcu_sync_is_idle() will return false, which 113 * tells readers to stay off their fastpaths. A later call to 114 * rcu_sync_exit() re-enables reader slowpaths. 115 * 116 * When called in isolation, rcu_sync_enter() must wait for a grace 117 * period, however, closely spaced calls to rcu_sync_enter() can 118 * optimize away the grace-period wait via a state machine implemented 119 * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). 120 */ 121 void rcu_sync_enter(struct rcu_sync *rsp) 122 { 123 int gp_state; 124 125 spin_lock_irq(&rsp->rss_lock); 126 gp_state = rsp->gp_state; 127 if (gp_state == GP_IDLE) { 128 WRITE_ONCE(rsp->gp_state, GP_ENTER); 129 WARN_ON_ONCE(rsp->gp_count); 130 /* 131 * Note that we could simply do rcu_sync_call(rsp) here and 132 * avoid the "if (gp_state == GP_IDLE)" block below. 133 * 134 * However, synchronize_rcu() can be faster if rcu_expedited 135 * or rcu_blocking_is_gp() is true. 136 * 137 * Another reason is that we can't wait for rcu callback if 138 * we are called at early boot time but this shouldn't happen. 139 */ 140 } 141 rsp->gp_count++; 142 spin_unlock_irq(&rsp->rss_lock); 143 144 if (gp_state == GP_IDLE) { 145 /* 146 * See the comment above, this simply does the "synchronous" 147 * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED. 148 */ 149 synchronize_rcu(); 150 rcu_sync_func(&rsp->cb_head); 151 /* Not really needed, wait_event() would see GP_PASSED. */ 152 return; 153 } 154 155 wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED); 156 } 157 158 /** 159 * rcu_sync_exit() - Allow readers back onto fast path after grace period 160 * @rsp: Pointer to rcu_sync structure to use for synchronization 161 * 162 * This function is used by updaters who have completed, and can therefore 163 * now allow readers to make use of their fastpaths after a grace period 164 * has elapsed. After this grace period has completed, all subsequent 165 * calls to rcu_sync_is_idle() will return true, which tells readers that 166 * they can once again use their fastpaths. 167 */ 168 void rcu_sync_exit(struct rcu_sync *rsp) 169 { 170 WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); 171 WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0); 172 173 spin_lock_irq(&rsp->rss_lock); 174 if (!--rsp->gp_count) { 175 if (rsp->gp_state == GP_PASSED) { 176 WRITE_ONCE(rsp->gp_state, GP_EXIT); 177 rcu_sync_call(rsp); 178 } else if (rsp->gp_state == GP_EXIT) { 179 WRITE_ONCE(rsp->gp_state, GP_REPLAY); 180 } 181 } 182 spin_unlock_irq(&rsp->rss_lock); 183 } 184 185 /** 186 * rcu_sync_dtor() - Clean up an rcu_sync structure 187 * @rsp: Pointer to rcu_sync structure to be cleaned up 188 */ 189 void rcu_sync_dtor(struct rcu_sync *rsp) 190 { 191 int gp_state; 192 193 WARN_ON_ONCE(READ_ONCE(rsp->gp_count)); 194 WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); 195 196 spin_lock_irq(&rsp->rss_lock); 197 if (rsp->gp_state == GP_REPLAY) 198 WRITE_ONCE(rsp->gp_state, GP_EXIT); 199 gp_state = rsp->gp_state; 200 spin_unlock_irq(&rsp->rss_lock); 201 202 if (gp_state != GP_IDLE) { 203 rcu_barrier(); 204 WARN_ON_ONCE(rsp->gp_state != GP_IDLE); 205 } 206 } 207