// SPDX-License-Identifier: GPL-2.0+ /* * RCU-based infrastructure for lightweight reader-writer locking * * Copyright (c) 2015, Red Hat, Inc. * * Author: Oleg Nesterov <oleg@redhat.com> */ #include <linux/rcu_sync.h> #include <linux/sched.h> enum { GP_IDLE = 0, GP_ENTER, GP_PASSED, GP_EXIT, GP_REPLAY }; #define rss_lock gp_wait.lock /** * rcu_sync_init() - Initialize an rcu_sync structure * @rsp: Pointer to rcu_sync structure to be initialized */ void rcu_sync_init(struct rcu_sync *rsp) { memset(rsp, 0, sizeof(*rsp)); init_waitqueue_head(&rsp->gp_wait); } static void rcu_sync_func(struct rcu_head *rhp); static void rcu_sync_call(struct rcu_sync *rsp) { call_rcu_hurry(&rsp->cb_head, rcu_sync_func); } /** * rcu_sync_func() - Callback function managing reader access to fastpath * @rhp: Pointer to rcu_head in rcu_sync structure to use for synchronization * * This function is passed to call_rcu() function by rcu_sync_enter() and * rcu_sync_exit(), so that it is invoked after a grace period following the * that invocation of enter/exit. * * If it is called by rcu_sync_enter() it signals that all the readers were * switched onto slow path. * * If it is called by rcu_sync_exit() it takes action based on events that * have taken place in the meantime, so that closely spaced rcu_sync_enter() * and rcu_sync_exit() pairs need not wait for a grace period. * * If another rcu_sync_enter() is invoked before the grace period * ended, reset state to allow the next rcu_sync_exit() to let the * readers back onto their fastpaths (after a grace period). If both * another rcu_sync_enter() and its matching rcu_sync_exit() are invoked * before the grace period ended, re-invoke call_rcu() on behalf of that * rcu_sync_exit(). Otherwise, set all state back to idle so that readers * can again use their fastpaths. */ static void rcu_sync_func(struct rcu_head *rhp) { struct rcu_sync *rsp = container_of(rhp, struct rcu_sync, cb_head); unsigned long flags; WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); spin_lock_irqsave(&rsp->rss_lock, flags); if (rsp->gp_count) { /* * We're at least a GP after the GP_IDLE->GP_ENTER transition. */ WRITE_ONCE(rsp->gp_state, GP_PASSED); wake_up_locked(&rsp->gp_wait); } else if (rsp->gp_state == GP_REPLAY) { /* * A new rcu_sync_exit() has happened; requeue the callback to * catch a later GP. */ WRITE_ONCE(rsp->gp_state, GP_EXIT); rcu_sync_call(rsp); } else { /* * We're at least a GP after the last rcu_sync_exit(); everybody * will now have observed the write side critical section. * Let 'em rip! */ WRITE_ONCE(rsp->gp_state, GP_IDLE); } spin_unlock_irqrestore(&rsp->rss_lock, flags); } /** * rcu_sync_enter() - Force readers onto slowpath * @rsp: Pointer to rcu_sync structure to use for synchronization * * This function is used by updaters who need readers to make use of * a slowpath during the update. After this function returns, all * subsequent calls to rcu_sync_is_idle() will return false, which * tells readers to stay off their fastpaths. A later call to * rcu_sync_exit() re-enables reader fastpaths. * * When called in isolation, rcu_sync_enter() must wait for a grace * period, however, closely spaced calls to rcu_sync_enter() can * optimize away the grace-period wait via a state machine implemented * by rcu_sync_enter(), rcu_sync_exit(), and rcu_sync_func(). */ void rcu_sync_enter(struct rcu_sync *rsp) { int gp_state; spin_lock_irq(&rsp->rss_lock); gp_state = rsp->gp_state; if (gp_state == GP_IDLE) { WRITE_ONCE(rsp->gp_state, GP_ENTER); WARN_ON_ONCE(rsp->gp_count); /* * Note that we could simply do rcu_sync_call(rsp) here and * avoid the "if (gp_state == GP_IDLE)" block below. * * However, synchronize_rcu() can be faster if rcu_expedited * or rcu_blocking_is_gp() is true. * * Another reason is that we can't wait for rcu callback if * we are called at early boot time but this shouldn't happen. */ } rsp->gp_count++; spin_unlock_irq(&rsp->rss_lock); if (gp_state == GP_IDLE) { /* * See the comment above, this simply does the "synchronous" * call_rcu(rcu_sync_func) which does GP_ENTER -> GP_PASSED. */ synchronize_rcu(); rcu_sync_func(&rsp->cb_head); /* Not really needed, wait_event() would see GP_PASSED. */ return; } wait_event(rsp->gp_wait, READ_ONCE(rsp->gp_state) >= GP_PASSED); } /** * rcu_sync_exit() - Allow readers back onto fast path after grace period * @rsp: Pointer to rcu_sync structure to use for synchronization * * This function is used by updaters who have completed, and can therefore * now allow readers to make use of their fastpaths after a grace period * has elapsed. After this grace period has completed, all subsequent * calls to rcu_sync_is_idle() will return true, which tells readers that * they can once again use their fastpaths. */ void rcu_sync_exit(struct rcu_sync *rsp) { WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_IDLE); spin_lock_irq(&rsp->rss_lock); WARN_ON_ONCE(rsp->gp_count == 0); if (!--rsp->gp_count) { if (rsp->gp_state == GP_PASSED) { WRITE_ONCE(rsp->gp_state, GP_EXIT); rcu_sync_call(rsp); } else if (rsp->gp_state == GP_EXIT) { WRITE_ONCE(rsp->gp_state, GP_REPLAY); } } spin_unlock_irq(&rsp->rss_lock); } /** * rcu_sync_dtor() - Clean up an rcu_sync structure * @rsp: Pointer to rcu_sync structure to be cleaned up */ void rcu_sync_dtor(struct rcu_sync *rsp) { int gp_state; WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); spin_lock_irq(&rsp->rss_lock); WARN_ON_ONCE(rsp->gp_count); if (rsp->gp_state == GP_REPLAY) WRITE_ONCE(rsp->gp_state, GP_EXIT); gp_state = rsp->gp_state; spin_unlock_irq(&rsp->rss_lock); if (gp_state != GP_IDLE) { rcu_barrier(); WARN_ON_ONCE(rsp->gp_state != GP_IDLE); } }
