/* * RCU-based infrastructure for lightweight reader-writer locking * * This program is free software; you can redistribute it and/or modify * it under the terms of the GNU General Public License as published by * the Free Software Foundation; either version 2 of the License, or * (at your option) any later version. * * This program is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the * GNU General Public License for more details. * * You should have received a copy of the GNU General Public License * along with this program; if not, you can access it online at * http://www.gnu.org/licenses/gpl-2.0.html. * * Copyright (c) 2015, Red Hat, Inc. * * Author: Oleg Nesterov <oleg@redhat.com> */ #include <linux/rcu_sync.h>#include <linux/sched.h> #ifdef CONFIG_PROVE_RCU#define __INIT_HELD(func)	.held = func,#else#define __INIT_HELD(func)#endif static const struct {	void (*sync)(void);	void (*call)(struct rcu_head *, void (*)(struct rcu_head *));	void (*wait)(void);#ifdef CONFIG_PROVE_RCU	int  (*held)(void);#endif} gp_ops[] = {	[RCU_SYNC] = {		.sync = synchronize_rcu,		.call = call_rcu,		.wait = rcu_barrier,		__INIT_HELD(rcu_read_lock_held)	},	[RCU_SCHED_SYNC] = {		.sync = synchronize_sched,		.call = call_rcu_sched,		.wait = rcu_barrier_sched,		__INIT_HELD(rcu_read_lock_sched_held)	},	[RCU_BH_SYNC] = {		.sync = synchronize_rcu_bh,		.call = call_rcu_bh,		.wait = rcu_barrier_bh,		__INIT_HELD(rcu_read_lock_bh_held)	},}; enum { GP_IDLE = 0, GP_PENDING, GP_PASSED };enum { CB_IDLE = 0, CB_PENDING, CB_REPLAY }; #define	rss_lock	gp_wait.lock #ifdef CONFIG_PROVE_RCUvoid rcu_sync_lockdep_assert(struct rcu_sync *rsp){	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),			 "suspicious rcu_sync_is_idle() usage");}#endif /** * rcu_sync_init() - Initialize an rcu_sync structure * @rsp: Pointer to rcu_sync structure to be initialized * @type: Flavor of RCU with which to synchronize rcu_sync structure */void rcu_sync_init(struct rcu_sync *rsp, enum rcu_sync_type type){	memset(rsp, 0, sizeof(*rsp));	init_waitqueue_head(&rsp->gp_wait);	rsp->gp_type = type;} /** * 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){	bool need_wait, need_sync; 	spin_lock_irq(&rsp->rss_lock);	need_wait = rsp->gp_count++;	need_sync = rsp->gp_state == GP_IDLE;	if (need_sync)		rsp->gp_state = GP_PENDING;	spin_unlock_irq(&rsp->rss_lock); 	BUG_ON(need_wait && need_sync); 	if (need_sync) {		gp_ops[rsp->gp_type].sync();		rsp->gp_state = GP_PASSED;		wake_up_all(&rsp->gp_wait);	} else if (need_wait) {		wait_event(rsp->gp_wait, rsp->gp_state == GP_PASSED);	} else {		/*		 * Possible when there's a pending CB from a rcu_sync_exit().		 * Nobody has yet been allowed the 'fast' path and thus we can		 * avoid doing any sync(). The callback will get 'dropped'.		 */		BUG_ON(rsp->gp_state != GP_PASSED);	}} /** * rcu_sync_func() - Callback function managing reader access to fastpath * @rsp: Pointer to rcu_sync structure to use for synchronization * * This function is passed to one of the call_rcu() functions by * rcu_sync_exit(), so that it is invoked after a grace period following the * that invocation of 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 *rcu){	struct rcu_sync *rsp = container_of(rcu, struct rcu_sync, cb_head);	unsigned long flags; 	BUG_ON(rsp->gp_state != GP_PASSED);	BUG_ON(rsp->cb_state == CB_IDLE); 	spin_lock_irqsave(&rsp->rss_lock, flags);	if (rsp->gp_count) {		/*		 * A new rcu_sync_begin() has happened; drop the callback.		 */		rsp->cb_state = CB_IDLE;	} else if (rsp->cb_state == CB_REPLAY) {		/*		 * A new rcu_sync_exit() has happened; requeue the callback		 * to catch a later GP.		 */		rsp->cb_state = CB_PENDING;		gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);	} else {		/*		 * We're at least a GP after rcu_sync_exit(); eveybody will now		 * have observed the write side critical section. Let 'em rip!.		 */		rsp->cb_state = CB_IDLE;		rsp->gp_state = GP_IDLE;	}	spin_unlock_irqrestore(&rsp->rss_lock, flags);} /** * rcu_sync_exit() - Allow readers back onto fast patch 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){	spin_lock_irq(&rsp->rss_lock);	if (!--rsp->gp_count) {		if (rsp->cb_state == CB_IDLE) {			rsp->cb_state = CB_PENDING;			gp_ops[rsp->gp_type].call(&rsp->cb_head, rcu_sync_func);		} else if (rsp->cb_state == CB_PENDING) {			rsp->cb_state = CB_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 cb_state; 	BUG_ON(rsp->gp_count); 	spin_lock_irq(&rsp->rss_lock);	if (rsp->cb_state == CB_REPLAY)		rsp->cb_state = CB_PENDING;	cb_state = rsp->cb_state;	spin_unlock_irq(&rsp->rss_lock); 	if (cb_state != CB_IDLE) {		gp_ops[rsp->gp_type].wait();		BUG_ON(rsp->cb_state != CB_IDLE);	}}