/*
   * 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_RCU

  void rcu_sync_lockdep_assert(struct rcu_sync *rsp)

  {

  	RCU_LOCKDEP_WARN(!gp_ops[rsp->gp_type].held(),
  			 "suspicious rcu_sync_is_idle() usage");

  }

  
  EXPORT_SYMBOL_GPL(rcu_sync_lockdep_assert);

  #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_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;
  }
  
  /**

   * 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

   * @rhp: Pointer to rcu_head in 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 *rhp)

  {

  	struct rcu_sync *rsp = container_of(rhp, 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);
  	}
  }