/*
   * Read-Copy Update mechanism for mutual exclusion
   *
   * 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, write to the Free Software
   * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
   *

   * Copyright IBM Corporation, 2001

   *
   * Authors: Dipankar Sarma <dipankar@in.ibm.com>
   *	    Manfred Spraul <manfred@colorfullife.com>
   * 
   * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
   * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
   * Papers:
   * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
   * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
   *
   * For detailed explanation of Read-Copy Update mechanism see -
   * 		http://lse.sourceforge.net/locking/rcupdate.html
   *
   */
  #include <linux/types.h>
  #include <linux/kernel.h>
  #include <linux/init.h>
  #include <linux/spinlock.h>
  #include <linux/smp.h>
  #include <linux/interrupt.h>
  #include <linux/sched.h>
  #include <asm/atomic.h>
  #include <linux/bitops.h>

  #include <linux/percpu.h>
  #include <linux/notifier.h>

  #include <linux/cpu.h>

  #include <linux/mutex.h>

  #include <linux/module.h>

  #include <linux/kernel_stat.h>

  enum rcu_barrier {
  	RCU_BARRIER_STD,
  	RCU_BARRIER_BH,
  	RCU_BARRIER_SCHED,
  };

  static DEFINE_PER_CPU(struct rcu_head, rcu_barrier_head) = {NULL};

  static atomic_t rcu_barrier_cpu_count;

  static DEFINE_MUTEX(rcu_barrier_mutex);

  static struct completion rcu_barrier_completion;

  int rcu_scheduler_active __read_mostly;

  static atomic_t rcu_migrate_type_count = ATOMIC_INIT(0);
  static struct rcu_head rcu_migrate_head[3];
  static DECLARE_WAIT_QUEUE_HEAD(rcu_migrate_wq);

  /*
   * Awaken the corresponding synchronize_rcu() instance now that a
   * grace period has elapsed.
   */

  void wakeme_after_rcu(struct rcu_head  *head)

  {

  	struct rcu_synchronize *rcu;
  
  	rcu = container_of(head, struct rcu_synchronize, head);
  	complete(&rcu->completion);

  }

  
  /**

   * synchronize_rcu - wait until a grace period has elapsed.

   *

   * Control will return to the caller some time after a full grace
   * period has elapsed, in other words after all currently executing RCU

   * read-side critical sections have completed.  RCU read-side critical
   * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
   * and may be nested.
   */

  void synchronize_rcu(void)
  {
  	struct rcu_synchronize rcu;

  
  	if (rcu_blocking_is_gp())
  		return;

  	init_completion(&rcu.completion);
  	/* Will wake me after RCU finished. */
  	call_rcu(&rcu.head, wakeme_after_rcu);
  	/* Wait for it. */
  	wait_for_completion(&rcu.completion);
  }

  EXPORT_SYMBOL_GPL(synchronize_rcu);

  static void rcu_barrier_callback(struct rcu_head *notused)
  {
  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
  		complete(&rcu_barrier_completion);
  }
  
  /*
   * Called with preemption disabled, and from cross-cpu IRQ context.
   */

  static void rcu_barrier_func(void *type)

  {
  	int cpu = smp_processor_id();

  	struct rcu_head *head = &per_cpu(rcu_barrier_head, cpu);

  	atomic_inc(&rcu_barrier_cpu_count);

  	switch ((enum rcu_barrier)type) {
  	case RCU_BARRIER_STD:
  		call_rcu(head, rcu_barrier_callback);
  		break;
  	case RCU_BARRIER_BH:
  		call_rcu_bh(head, rcu_barrier_callback);
  		break;
  	case RCU_BARRIER_SCHED:
  		call_rcu_sched(head, rcu_barrier_callback);
  		break;
  	}

  }

  static inline void wait_migrated_callbacks(void)
  {
  	wait_event(rcu_migrate_wq, !atomic_read(&rcu_migrate_type_count));
  }

  /*
   * Orchestrate the specified type of RCU barrier, waiting for all
   * RCU callbacks of the specified type to complete.

   */

  static void _rcu_barrier(enum rcu_barrier type)

  {
  	BUG_ON(in_interrupt());

  	/* Take cpucontrol mutex to protect against CPU hotplug */
  	mutex_lock(&rcu_barrier_mutex);

  	init_completion(&rcu_barrier_completion);

  	/*

  	 * Initialize rcu_barrier_cpu_count to 1, then invoke
  	 * rcu_barrier_func() on each CPU, so that each CPU also has
  	 * incremented rcu_barrier_cpu_count.  Only then is it safe to
  	 * decrement rcu_barrier_cpu_count -- otherwise the first CPU
  	 * might complete its grace period before all of the other CPUs
  	 * did their increment, causing this function to return too
  	 * early.

  	 */

  	atomic_set(&rcu_barrier_cpu_count, 1);

  	on_each_cpu(rcu_barrier_func, (void *)type, 1);

  	if (atomic_dec_and_test(&rcu_barrier_cpu_count))
  		complete(&rcu_barrier_completion);

  	wait_for_completion(&rcu_barrier_completion);

  	mutex_unlock(&rcu_barrier_mutex);

  	wait_migrated_callbacks();

  }

  
  /**
   * rcu_barrier - Wait until all in-flight call_rcu() callbacks complete.
   */
  void rcu_barrier(void)
  {
  	_rcu_barrier(RCU_BARRIER_STD);
  }

  EXPORT_SYMBOL_GPL(rcu_barrier);

  /**
   * rcu_barrier_bh - Wait until all in-flight call_rcu_bh() callbacks complete.
   */
  void rcu_barrier_bh(void)
  {
  	_rcu_barrier(RCU_BARRIER_BH);
  }
  EXPORT_SYMBOL_GPL(rcu_barrier_bh);
  
  /**
   * rcu_barrier_sched - Wait for in-flight call_rcu_sched() callbacks.
   */
  void rcu_barrier_sched(void)
  {
  	_rcu_barrier(RCU_BARRIER_SCHED);
  }
  EXPORT_SYMBOL_GPL(rcu_barrier_sched);

  static void rcu_migrate_callback(struct rcu_head *notused)
  {
  	if (atomic_dec_and_test(&rcu_migrate_type_count))
  		wake_up(&rcu_migrate_wq);
  }

  static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
  		unsigned long action, void *hcpu)
  {
  	if (action == CPU_DYING) {
  		/*
  		 * preempt_disable() in on_each_cpu() prevents stop_machine(),
  		 * so when "on_each_cpu(rcu_barrier_func, (void *)type, 1);"
  		 * returns, all online cpus have queued rcu_barrier_func(),
  		 * and the dead cpu(if it exist) queues rcu_migrate_callback()s.
  		 *
  		 * These callbacks ensure _rcu_barrier() waits for all
  		 * RCU callbacks of the specified type to complete.
  		 */
  		atomic_set(&rcu_migrate_type_count, 3);
  		call_rcu_bh(rcu_migrate_head, rcu_migrate_callback);
  		call_rcu_sched(rcu_migrate_head + 1, rcu_migrate_callback);
  		call_rcu(rcu_migrate_head + 2, rcu_migrate_callback);
  	} else if (action == CPU_POST_DEAD) {
  		/* rcu_migrate_head is protected by cpu_add_remove_lock */
  		wait_migrated_callbacks();
  	}
  
  	return NOTIFY_OK;
  }

  void __init rcu_init(void)
  {

  	__rcu_init();

  	hotcpu_notifier(rcu_barrier_cpu_hotplug, 0);

  }

  void rcu_scheduler_starting(void)
  {
  	WARN_ON(num_online_cpus() != 1);
  	WARN_ON(nr_context_switches() > 0);
  	rcu_scheduler_active = 1;
  }