// 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);} /** * rcu_sync_enter_start - Force readers onto slow path for multiple updates * @rsp: Pointer to rcu_sync structure to use for synchronization * * Must be called after rcu_sync_init() and before first use. * * Ensures rcu_sync_is_idle() returns false and rcu_sync_{enter,exit}() * pairs turn into NO-OPs. */void rcu_sync_enter_start(struct rcu_sync *rsp){	rsp->gp_count++;	rsp->gp_state = GP_PASSED;}  static void rcu_sync_func(struct rcu_head *rhp); static void rcu_sync_call(struct rcu_sync *rsp){	call_rcu(&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(); eveybody		 * 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 slowpaths. * * 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);	WARN_ON_ONCE(READ_ONCE(rsp->gp_count) == 0); 	spin_lock_irq(&rsp->rss_lock);	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_count));	WARN_ON_ONCE(READ_ONCE(rsp->gp_state) == GP_PASSED); 	spin_lock_irq(&rsp->rss_lock);	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);	}}