/*

   * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition

   * Internal non-public definitions that provide either classic

   * or preemptible semantics.

   *
   * 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 (c) 2010 Linaro

   *
   * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
   */

  #include <linux/kthread.h>

  #include <linux/module.h>

  #include <linux/debugfs.h>
  #include <linux/seq_file.h>

  /* Global control variables for rcupdate callback mechanism. */
  struct rcu_ctrlblk {
  	struct rcu_head *rcucblist;	/* List of pending callbacks (CBs). */
  	struct rcu_head **donetail;	/* ->next pointer of last "done" CB. */
  	struct rcu_head **curtail;	/* ->next pointer of last CB. */

  	RCU_TRACE(long qlen);		/* Number of pending CBs. */

  	RCU_TRACE(unsigned long gp_start); /* Start time for stalls. */
  	RCU_TRACE(unsigned long ticks_this_gp); /* Statistic for stalls. */
  	RCU_TRACE(unsigned long jiffies_stall); /* Jiffies at next stall. */

  	RCU_TRACE(char *name);		/* Name of RCU type. */

  };
  
  /* Definition for rcupdate control block. */
  static struct rcu_ctrlblk rcu_sched_ctrlblk = {
  	.donetail	= &rcu_sched_ctrlblk.rcucblist,
  	.curtail	= &rcu_sched_ctrlblk.rcucblist,

  	RCU_TRACE(.name = "rcu_sched")

  };
  
  static struct rcu_ctrlblk rcu_bh_ctrlblk = {
  	.donetail	= &rcu_bh_ctrlblk.rcucblist,
  	.curtail	= &rcu_bh_ctrlblk.rcucblist,

  	RCU_TRACE(.name = "rcu_bh")

  };
  
  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  int rcu_scheduler_active __read_mostly;
  EXPORT_SYMBOL_GPL(rcu_scheduler_active);
  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

  #ifdef CONFIG_RCU_TRACE
  
  static void check_cpu_stall(struct rcu_ctrlblk *rcp)
  {
  	unsigned long j;
  	unsigned long js;
  
  	if (rcu_cpu_stall_suppress)
  		return;
  	rcp->ticks_this_gp++;
  	j = jiffies;
  	js = rcp->jiffies_stall;
  	if (*rcp->curtail && ULONG_CMP_GE(j, js)) {
  		pr_err("INFO: %s stall on CPU (%lu ticks this GP) idle=%llx (t=%lu jiffies q=%ld)
  ",
  		       rcp->name, rcp->ticks_this_gp, rcu_dynticks_nesting,
  		       jiffies - rcp->gp_start, rcp->qlen);
  		dump_stack();
  	}
  	if (*rcp->curtail && ULONG_CMP_GE(j, js))
  		rcp->jiffies_stall = jiffies +
  			3 * rcu_jiffies_till_stall_check() + 3;
  	else if (ULONG_CMP_GE(j, js))
  		rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
  }
  
  static void check_cpu_stall_preempt(void);
  
  #endif /* #ifdef CONFIG_RCU_TRACE */
  
  static void reset_cpu_stall_ticks(struct rcu_ctrlblk *rcp)
  {
  #ifdef CONFIG_RCU_TRACE
  	rcp->ticks_this_gp = 0;
  	rcp->gp_start = jiffies;
  	rcp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check();
  #endif /* #ifdef CONFIG_RCU_TRACE */
  }
  
  static void check_cpu_stalls(void)
  {
  	RCU_TRACE(check_cpu_stall(&rcu_bh_ctrlblk));
  	RCU_TRACE(check_cpu_stall(&rcu_sched_ctrlblk));
  	RCU_TRACE(check_cpu_stall_preempt());
  }

  #ifdef CONFIG_TINY_PREEMPT_RCU
  
  #include <linux/delay.h>

  /* Global control variables for preemptible RCU. */
  struct rcu_preempt_ctrlblk {
  	struct rcu_ctrlblk rcb;	/* curtail: ->next ptr of last CB for GP. */
  	struct rcu_head **nexttail;
  				/* Tasks blocked in a preemptible RCU */
  				/*  read-side critical section while an */
  				/*  preemptible-RCU grace period is in */
  				/*  progress must wait for a later grace */
  				/*  period.  This pointer points to the */
  				/*  ->next pointer of the last task that */
  				/*  must wait for a later grace period, or */
  				/*  to &->rcb.rcucblist if there is no */
  				/*  such task. */
  	struct list_head blkd_tasks;
  				/* Tasks blocked in RCU read-side critical */
  				/*  section.  Tasks are placed at the head */
  				/*  of this list and age towards the tail. */
  	struct list_head *gp_tasks;
  				/* Pointer to the first task blocking the */
  				/*  current grace period, or NULL if there */

  				/*  is no such task. */

  	struct list_head *exp_tasks;
  				/* Pointer to first task blocking the */
  				/*  current expedited grace period, or NULL */
  				/*  if there is no such task.  If there */
  				/*  is no current expedited grace period, */
  				/*  then there cannot be any such task. */

  #ifdef CONFIG_RCU_BOOST
  	struct list_head *boost_tasks;
  				/* Pointer to first task that needs to be */
  				/*  priority-boosted, or NULL if no priority */
  				/*  boosting is needed.  If there is no */
  				/*  current or expedited grace period, there */
  				/*  can be no such task. */
  #endif /* #ifdef CONFIG_RCU_BOOST */

  	u8 gpnum;		/* Current grace period. */
  	u8 gpcpu;		/* Last grace period blocked by the CPU. */
  	u8 completed;		/* Last grace period completed. */
  				/*  If all three are equal, RCU is idle. */

  #ifdef CONFIG_RCU_BOOST

  	unsigned long boost_time; /* When to start boosting (jiffies) */

  #endif /* #ifdef CONFIG_RCU_BOOST */
  #ifdef CONFIG_RCU_TRACE
  	unsigned long n_grace_periods;
  #ifdef CONFIG_RCU_BOOST
  	unsigned long n_tasks_boosted;

  				/* Total number of tasks boosted. */

  	unsigned long n_exp_boosts;

  				/* Number of tasks boosted for expedited GP. */

  	unsigned long n_normal_boosts;

  				/* Number of tasks boosted for normal GP. */
  	unsigned long n_balk_blkd_tasks;
  				/* Refused to boost: no blocked tasks. */
  	unsigned long n_balk_exp_gp_tasks;
  				/* Refused to boost: nothing blocking GP. */
  	unsigned long n_balk_boost_tasks;
  				/* Refused to boost: already boosting. */
  	unsigned long n_balk_notyet;
  				/* Refused to boost: not yet time. */
  	unsigned long n_balk_nos;
  				/* Refused to boost: not sure why, though. */
  				/*  This can happen due to race conditions. */

  #endif /* #ifdef CONFIG_RCU_BOOST */
  #endif /* #ifdef CONFIG_RCU_TRACE */

  };
  
  static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
  	.rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
  	.rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
  	.nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
  	.blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),

  	RCU_TRACE(.rcb.name = "rcu_preempt")

  };
  
  static int rcu_preempted_readers_exp(void);
  static void rcu_report_exp_done(void);
  
  /*
   * Return true if the CPU has not yet responded to the current grace period.
   */

  static int rcu_cpu_blocking_cur_gp(void)

  {
  	return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
  }
  
  /*
   * Check for a running RCU reader.  Because there is only one CPU,
   * there can be but one running RCU reader at a time.  ;-)

   *
   * Returns zero if there are no running readers.  Returns a positive
   * number if there is at least one reader within its RCU read-side
   * critical section.  Returns a negative number if an outermost reader
   * is in the midst of exiting from its RCU read-side critical section
   *
   * Returns zero if there are no running readers.  Returns a positive
   * number if there is at least one reader within its RCU read-side
   * critical section.  Returns a negative number if an outermost reader
   * is in the midst of exiting from its RCU read-side critical section.

   */
  static int rcu_preempt_running_reader(void)
  {
  	return current->rcu_read_lock_nesting;
  }
  
  /*
   * Check for preempted RCU readers blocking any grace period.
   * If the caller needs a reliable answer, it must disable hard irqs.
   */
  static int rcu_preempt_blocked_readers_any(void)
  {
  	return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
  }
  
  /*
   * Check for preempted RCU readers blocking the current grace period.
   * If the caller needs a reliable answer, it must disable hard irqs.
   */
  static int rcu_preempt_blocked_readers_cgp(void)
  {
  	return rcu_preempt_ctrlblk.gp_tasks != NULL;
  }
  
  /*
   * Return true if another preemptible-RCU grace period is needed.
   */
  static int rcu_preempt_needs_another_gp(void)
  {
  	return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
  }
  
  /*
   * Return true if a preemptible-RCU grace period is in progress.
   * The caller must disable hardirqs.
   */
  static int rcu_preempt_gp_in_progress(void)
  {
  	return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
  }
  
  /*

   * Advance a ->blkd_tasks-list pointer to the next entry, instead
   * returning NULL if at the end of the list.
   */
  static struct list_head *rcu_next_node_entry(struct task_struct *t)
  {
  	struct list_head *np;
  
  	np = t->rcu_node_entry.next;
  	if (np == &rcu_preempt_ctrlblk.blkd_tasks)
  		np = NULL;
  	return np;
  }

  #ifdef CONFIG_RCU_TRACE
  
  #ifdef CONFIG_RCU_BOOST
  static void rcu_initiate_boost_trace(void);

  #endif /* #ifdef CONFIG_RCU_BOOST */
  
  /*
   * Dump additional statistice for TINY_PREEMPT_RCU.
   */
  static void show_tiny_preempt_stats(struct seq_file *m)
  {
  	seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c
  ",
  		   rcu_preempt_ctrlblk.rcb.qlen,
  		   rcu_preempt_ctrlblk.n_grace_periods,
  		   rcu_preempt_ctrlblk.gpnum,
  		   rcu_preempt_ctrlblk.gpcpu,
  		   rcu_preempt_ctrlblk.completed,
  		   "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
  		   "N."[!rcu_preempt_ctrlblk.gp_tasks],
  		   "E."[!rcu_preempt_ctrlblk.exp_tasks]);
  #ifdef CONFIG_RCU_BOOST

  	seq_printf(m, "%sttb=%c ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x
  ",
  		   "             ",
  		   "B."[!rcu_preempt_ctrlblk.boost_tasks],

  		   rcu_preempt_ctrlblk.n_tasks_boosted,
  		   rcu_preempt_ctrlblk.n_exp_boosts,
  		   rcu_preempt_ctrlblk.n_normal_boosts,
  		   (int)(jiffies & 0xffff),
  		   (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));

  	seq_printf(m, "%s: nt=%lu egt=%lu bt=%lu ny=%lu nos=%lu
  ",
  		   "             balk",
  		   rcu_preempt_ctrlblk.n_balk_blkd_tasks,
  		   rcu_preempt_ctrlblk.n_balk_exp_gp_tasks,
  		   rcu_preempt_ctrlblk.n_balk_boost_tasks,
  		   rcu_preempt_ctrlblk.n_balk_notyet,
  		   rcu_preempt_ctrlblk.n_balk_nos);

  #endif /* #ifdef CONFIG_RCU_BOOST */
  }
  
  #endif /* #ifdef CONFIG_RCU_TRACE */

  #ifdef CONFIG_RCU_BOOST
  
  #include "rtmutex_common.h"

  #define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO
  
  /* Controls for rcu_kthread() kthread. */
  static struct task_struct *rcu_kthread_task;
  static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
  static unsigned long have_rcu_kthread_work;

  /*
   * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
   * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
   */
  static int rcu_boost(void)
  {
  	unsigned long flags;
  	struct rt_mutex mtx;

  	struct task_struct *t;

  	struct list_head *tb;

  	if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
  	    rcu_preempt_ctrlblk.exp_tasks == NULL)

  		return 0;  /* Nothing to boost. */

  	local_irq_save(flags);

  
  	/*
  	 * Recheck with irqs disabled: all tasks in need of boosting
  	 * might exit their RCU read-side critical sections on their own
  	 * if we are preempted just before disabling irqs.
  	 */
  	if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
  	    rcu_preempt_ctrlblk.exp_tasks == NULL) {

  		local_irq_restore(flags);

  		return 0;
  	}
  
  	/*
  	 * Preferentially boost tasks blocking expedited grace periods.
  	 * This cannot starve the normal grace periods because a second
  	 * expedited grace period must boost all blocked tasks, including
  	 * those blocking the pre-existing normal grace period.
  	 */
  	if (rcu_preempt_ctrlblk.exp_tasks != NULL) {
  		tb = rcu_preempt_ctrlblk.exp_tasks;
  		RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
  	} else {
  		tb = rcu_preempt_ctrlblk.boost_tasks;
  		RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
  	}
  	RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);
  
  	/*
  	 * We boost task t by manufacturing an rt_mutex that appears to
  	 * be held by task t.  We leave a pointer to that rt_mutex where
  	 * task t can find it, and task t will release the mutex when it
  	 * exits its outermost RCU read-side critical section.  Then
  	 * simply acquiring this artificial rt_mutex will boost task
  	 * t's priority.  (Thanks to tglx for suggesting this approach!)
  	 */
  	t = container_of(tb, struct task_struct, rcu_node_entry);

  	rt_mutex_init_proxy_locked(&mtx, t);
  	t->rcu_boost_mutex = &mtx;

  	local_irq_restore(flags);

  	rt_mutex_lock(&mtx);

  	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */

  	return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
  	       ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;

  }
  
  /*
   * Check to see if it is now time to start boosting RCU readers blocking
   * the current grace period, and, if so, tell the rcu_kthread_task to
   * start boosting them.  If there is an expedited boost in progress,
   * we wait for it to complete.

   *
   * If there are no blocked readers blocking the current grace period,
   * return 0 to let the caller know, otherwise return 1.  Note that this
   * return value is independent of whether or not boosting was done.

   */

  static int rcu_initiate_boost(void)

  {

  	if (!rcu_preempt_blocked_readers_cgp() &&
  	    rcu_preempt_ctrlblk.exp_tasks == NULL) {
  		RCU_TRACE(rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++);

  		return 0;
  	}

  	if (rcu_preempt_ctrlblk.exp_tasks != NULL ||
  	    (rcu_preempt_ctrlblk.gp_tasks != NULL &&
  	     rcu_preempt_ctrlblk.boost_tasks == NULL &&
  	     ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))) {
  		if (rcu_preempt_ctrlblk.exp_tasks == NULL)
  			rcu_preempt_ctrlblk.boost_tasks =
  				rcu_preempt_ctrlblk.gp_tasks;

  		invoke_rcu_callbacks();

  	} else {

  		RCU_TRACE(rcu_initiate_boost_trace());

  	}

  	return 1;

  }

  #define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)

  
  /*
   * Do priority-boost accounting for the start of a new grace period.
   */
  static void rcu_preempt_boost_start_gp(void)
  {
  	rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;

  }
  
  #else /* #ifdef CONFIG_RCU_BOOST */
  
  /*

   * If there is no RCU priority boosting, we don't initiate boosting,
   * but we do indicate whether there are blocked readers blocking the
   * current grace period.

   */

  static int rcu_initiate_boost(void)

  {

  	return rcu_preempt_blocked_readers_cgp();

  }
  
  /*

   * If there is no RCU priority boosting, nothing to do at grace-period start.
   */
  static void rcu_preempt_boost_start_gp(void)
  {
  }
  
  #endif /* else #ifdef CONFIG_RCU_BOOST */
  
  /*

   * Record a preemptible-RCU quiescent state for the specified CPU.  Note
   * that this just means that the task currently running on the CPU is
   * in a quiescent state.  There might be any number of tasks blocked
   * while in an RCU read-side critical section.
   *
   * Unlike the other rcu_*_qs() functions, callers to this function
   * must disable irqs in order to protect the assignment to
   * ->rcu_read_unlock_special.
   *
   * Because this is a single-CPU implementation, the only way a grace
   * period can end is if the CPU is in a quiescent state.  The reason is
   * that a blocked preemptible-RCU reader can exit its critical section
   * only if the CPU is running it at the time.  Therefore, when the
   * last task blocking the current grace period exits its RCU read-side
   * critical section, neither the CPU nor blocked tasks will be stopping
   * the current grace period.  (In contrast, SMP implementations
   * might have CPUs running in RCU read-side critical sections that
   * block later grace periods -- but this is not possible given only
   * one CPU.)
   */
  static void rcu_preempt_cpu_qs(void)
  {
  	/* Record both CPU and task as having responded to current GP. */
  	rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
  	current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;

  	/* If there is no GP then there is nothing more to do.  */

  	if (!rcu_preempt_gp_in_progress())

  		return;

  	/*

  	 * Check up on boosting.  If there are readers blocking the

  	 * current grace period, leave.
  	 */
  	if (rcu_initiate_boost())

  		return;

  
  	/* Advance callbacks. */
  	rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
  	rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
  	rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;
  
  	/* If there are no blocked readers, next GP is done instantly. */
  	if (!rcu_preempt_blocked_readers_any())
  		rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;

  	/* If there are done callbacks, cause them to be invoked. */

  	if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)

  		invoke_rcu_callbacks();

  }
  
  /*
   * Start a new RCU grace period if warranted.  Hard irqs must be disabled.
   */
  static void rcu_preempt_start_gp(void)
  {
  	if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {
  
  		/* Official start of GP. */
  		rcu_preempt_ctrlblk.gpnum++;

  		RCU_TRACE(rcu_preempt_ctrlblk.n_grace_periods++);

  		reset_cpu_stall_ticks(&rcu_preempt_ctrlblk.rcb);

  
  		/* Any blocked RCU readers block new GP. */
  		if (rcu_preempt_blocked_readers_any())
  			rcu_preempt_ctrlblk.gp_tasks =
  				rcu_preempt_ctrlblk.blkd_tasks.next;

  		/* Set up for RCU priority boosting. */
  		rcu_preempt_boost_start_gp();

  		/* If there is no running reader, CPU is done with GP. */
  		if (!rcu_preempt_running_reader())
  			rcu_preempt_cpu_qs();
  	}
  }
  
  /*
   * We have entered the scheduler, and the current task might soon be
   * context-switched away from.  If this task is in an RCU read-side
   * critical section, we will no longer be able to rely on the CPU to
   * record that fact, so we enqueue the task on the blkd_tasks list.
   * If the task started after the current grace period began, as recorded
   * by ->gpcpu, we enqueue at the beginning of the list.  Otherwise
   * before the element referenced by ->gp_tasks (or at the tail if
   * ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
   * The task will dequeue itself when it exits the outermost enclosing
   * RCU read-side critical section.  Therefore, the current grace period
   * cannot be permitted to complete until the ->gp_tasks pointer becomes
   * NULL.
   *
   * Caller must disable preemption.
   */
  void rcu_preempt_note_context_switch(void)
  {
  	struct task_struct *t = current;
  	unsigned long flags;
  
  	local_irq_save(flags); /* must exclude scheduler_tick(). */

  	if (rcu_preempt_running_reader() > 0 &&

  	    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {
  
  		/* Possibly blocking in an RCU read-side critical section. */
  		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
  
  		/*
  		 * If this CPU has already checked in, then this task
  		 * will hold up the next grace period rather than the
  		 * current grace period.  Queue the task accordingly.
  		 * If the task is queued for the current grace period
  		 * (i.e., this CPU has not yet passed through a quiescent
  		 * state for the current grace period), then as long
  		 * as that task remains queued, the current grace period
  		 * cannot end.
  		 */
  		list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);

  		if (rcu_cpu_blocking_cur_gp())

  			rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;

  	} else if (rcu_preempt_running_reader() < 0 &&
  		   t->rcu_read_unlock_special) {
  		/*
  		 * Complete exit from RCU read-side critical section on
  		 * behalf of preempted instance of __rcu_read_unlock().
  		 */
  		rcu_read_unlock_special(t);

  	}
  
  	/*
  	 * Either we were not in an RCU read-side critical section to
  	 * begin with, or we have now recorded that critical section
  	 * globally.  Either way, we can now note a quiescent state
  	 * for this CPU.  Again, if we were in an RCU read-side critical
  	 * section, and if that critical section was blocking the current
  	 * grace period, then the fact that the task has been enqueued
  	 * means that current grace period continues to be blocked.
  	 */
  	rcu_preempt_cpu_qs();
  	local_irq_restore(flags);
  }
  
  /*

   * Handle special cases during rcu_read_unlock(), such as needing to
   * notify RCU core processing or task having blocked during the RCU
   * read-side critical section.
   */

  void rcu_read_unlock_special(struct task_struct *t)

  {
  	int empty;
  	int empty_exp;
  	unsigned long flags;
  	struct list_head *np;

  #ifdef CONFIG_RCU_BOOST
  	struct rt_mutex *rbmp = NULL;
  #endif /* #ifdef CONFIG_RCU_BOOST */

  	int special;
  
  	/*
  	 * NMI handlers cannot block and cannot safely manipulate state.
  	 * They therefore cannot possibly be special, so just leave.
  	 */
  	if (in_nmi())
  		return;
  
  	local_irq_save(flags);
  
  	/*
  	 * If RCU core is waiting for this CPU to exit critical section,
  	 * let it know that we have done so.
  	 */
  	special = t->rcu_read_unlock_special;
  	if (special & RCU_READ_UNLOCK_NEED_QS)
  		rcu_preempt_cpu_qs();
  
  	/* Hardware IRQ handlers cannot block. */

  	if (in_irq() || in_serving_softirq()) {

  		local_irq_restore(flags);
  		return;
  	}
  
  	/* Clean up if blocked during RCU read-side critical section. */
  	if (special & RCU_READ_UNLOCK_BLOCKED) {
  		t->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_BLOCKED;
  
  		/*
  		 * Remove this task from the ->blkd_tasks list and adjust
  		 * any pointers that might have been referencing it.
  		 */
  		empty = !rcu_preempt_blocked_readers_cgp();
  		empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;

  		np = rcu_next_node_entry(t);

  		list_del_init(&t->rcu_node_entry);

  		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
  			rcu_preempt_ctrlblk.gp_tasks = np;
  		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
  			rcu_preempt_ctrlblk.exp_tasks = np;

  #ifdef CONFIG_RCU_BOOST
  		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
  			rcu_preempt_ctrlblk.boost_tasks = np;
  #endif /* #ifdef CONFIG_RCU_BOOST */

  
  		/*
  		 * If this was the last task on the current list, and if
  		 * we aren't waiting on the CPU, report the quiescent state
  		 * and start a new grace period if needed.
  		 */
  		if (!empty && !rcu_preempt_blocked_readers_cgp()) {
  			rcu_preempt_cpu_qs();
  			rcu_preempt_start_gp();
  		}
  
  		/*
  		 * If this was the last task on the expedited lists,
  		 * then we need wake up the waiting task.
  		 */
  		if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
  			rcu_report_exp_done();
  	}

  #ifdef CONFIG_RCU_BOOST
  	/* Unboost self if was boosted. */

  	if (t->rcu_boost_mutex != NULL) {
  		rbmp = t->rcu_boost_mutex;

  		t->rcu_boost_mutex = NULL;

  		rt_mutex_unlock(rbmp);

  	}
  #endif /* #ifdef CONFIG_RCU_BOOST */

  	local_irq_restore(flags);
  }
  
  /*

   * Check for a quiescent state from the current CPU.  When a task blocks,
   * the task is recorded in the rcu_preempt_ctrlblk structure, which is
   * checked elsewhere.  This is called from the scheduling-clock interrupt.
   *
   * Caller must disable hard irqs.
   */
  static void rcu_preempt_check_callbacks(void)
  {
  	struct task_struct *t = current;

  	if (rcu_preempt_gp_in_progress() &&
  	    (!rcu_preempt_running_reader() ||
  	     !rcu_cpu_blocking_cur_gp()))

  		rcu_preempt_cpu_qs();
  	if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
  	    rcu_preempt_ctrlblk.rcb.donetail)

  		invoke_rcu_callbacks();

  	if (rcu_preempt_gp_in_progress() &&
  	    rcu_cpu_blocking_cur_gp() &&

  	    rcu_preempt_running_reader() > 0)

  		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
  }
  
  /*
   * TINY_PREEMPT_RCU has an extra callback-list tail pointer to

   * update, so this is invoked from rcu_process_callbacks() to

   * handle that case.  Of course, it is invoked for all flavors of
   * RCU, but RCU callbacks can appear only on one of the lists, and
   * neither ->nexttail nor ->donetail can possibly be NULL, so there
   * is no need for an explicit check.
   */
  static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
  {
  	if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
  		rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
  }
  
  /*
   * Process callbacks for preemptible RCU.
   */
  static void rcu_preempt_process_callbacks(void)
  {

  	__rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);

  }
  
  /*
   * Queue a preemptible -RCU callback for invocation after a grace period.
   */
  void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
  {
  	unsigned long flags;
  
  	debug_rcu_head_queue(head);
  	head->func = func;
  	head->next = NULL;
  
  	local_irq_save(flags);
  	*rcu_preempt_ctrlblk.nexttail = head;
  	rcu_preempt_ctrlblk.nexttail = &head->next;

  	RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);

  	rcu_preempt_start_gp();  /* checks to see if GP needed. */
  	local_irq_restore(flags);
  }
  EXPORT_SYMBOL_GPL(call_rcu);

  /*
   * 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)
  {

  	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
  			   !lock_is_held(&rcu_lock_map) &&
  			   !lock_is_held(&rcu_sched_lock_map),
  			   "Illegal synchronize_rcu() in RCU read-side critical section");

  #ifdef CONFIG_DEBUG_LOCK_ALLOC
  	if (!rcu_scheduler_active)
  		return;
  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
  
  	WARN_ON_ONCE(rcu_preempt_running_reader());
  	if (!rcu_preempt_blocked_readers_any())
  		return;
  
  	/* Once we get past the fastpath checks, same code as rcu_barrier(). */

  	if (rcu_expedited)
  		synchronize_rcu_expedited();
  	else
  		rcu_barrier();

  }
  EXPORT_SYMBOL_GPL(synchronize_rcu);
  
  static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
  static unsigned long sync_rcu_preempt_exp_count;
  static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);
  
  /*
   * Return non-zero if there are any tasks in RCU read-side critical
   * sections blocking the current preemptible-RCU expedited grace period.
   * If there is no preemptible-RCU expedited grace period currently in
   * progress, returns zero unconditionally.
   */
  static int rcu_preempted_readers_exp(void)
  {
  	return rcu_preempt_ctrlblk.exp_tasks != NULL;
  }
  
  /*
   * Report the exit from RCU read-side critical section for the last task
   * that queued itself during or before the current expedited preemptible-RCU
   * grace period.
   */
  static void rcu_report_exp_done(void)
  {
  	wake_up(&sync_rcu_preempt_exp_wq);
  }
  
  /*
   * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
   * is to rely in the fact that there is but one CPU, and that it is
   * illegal for a task to invoke synchronize_rcu_expedited() while in a
   * preemptible-RCU read-side critical section.  Therefore, any such
   * critical sections must correspond to blocked tasks, which must therefore
   * be on the ->blkd_tasks list.  So just record the current head of the
   * list in the ->exp_tasks pointer, and wait for all tasks including and
   * after the task pointed to by ->exp_tasks to drain.
   */
  void synchronize_rcu_expedited(void)
  {
  	unsigned long flags;
  	struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
  	unsigned long snap;
  
  	barrier(); /* ensure prior action seen before grace period. */
  
  	WARN_ON_ONCE(rcu_preempt_running_reader());
  
  	/*
  	 * Acquire lock so that there is only one preemptible RCU grace
  	 * period in flight.  Of course, if someone does the expedited
  	 * grace period for us while we are acquiring the lock, just leave.
  	 */
  	snap = sync_rcu_preempt_exp_count + 1;
  	mutex_lock(&sync_rcu_preempt_exp_mutex);
  	if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
  		goto unlock_mb_ret; /* Others did our work for us. */
  
  	local_irq_save(flags);
  
  	/*
  	 * All RCU readers have to already be on blkd_tasks because
  	 * we cannot legally be executing in an RCU read-side critical
  	 * section.
  	 */
  
  	/* Snapshot current head of ->blkd_tasks list. */
  	rpcp->exp_tasks = rpcp->blkd_tasks.next;
  	if (rpcp->exp_tasks == &rpcp->blkd_tasks)
  		rpcp->exp_tasks = NULL;

  
  	/* Wait for tail of ->blkd_tasks list to drain. */

  	if (!rcu_preempted_readers_exp()) {

  		local_irq_restore(flags);

  	} else {

  		rcu_initiate_boost();
  		local_irq_restore(flags);

  		wait_event(sync_rcu_preempt_exp_wq,
  			   !rcu_preempted_readers_exp());

  	}

  
  	/* Clean up and exit. */
  	barrier(); /* ensure expedited GP seen before counter increment. */
  	sync_rcu_preempt_exp_count++;
  unlock_mb_ret:
  	mutex_unlock(&sync_rcu_preempt_exp_mutex);
  	barrier(); /* ensure subsequent action seen after grace period. */
  }
  EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
  
  /*
   * Does preemptible RCU need the CPU to stay out of dynticks mode?
   */
  int rcu_preempt_needs_cpu(void)
  {

  	return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
  }

  #else /* #ifdef CONFIG_TINY_PREEMPT_RCU */

  #ifdef CONFIG_RCU_TRACE
  
  /*
   * Because preemptible RCU does not exist, it is not necessary to
   * dump out its statistics.
   */
  static void show_tiny_preempt_stats(struct seq_file *m)
  {
  }
  
  #endif /* #ifdef CONFIG_RCU_TRACE */

  /*
   * Because preemptible RCU does not exist, it never has any callbacks
   * to check.
   */
  static void rcu_preempt_check_callbacks(void)
  {
  }
  
  /*
   * Because preemptible RCU does not exist, it never has any callbacks
   * to remove.
   */
  static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
  {
  }
  
  /*
   * Because preemptible RCU does not exist, it never has any callbacks
   * to process.
   */
  static void rcu_preempt_process_callbacks(void)
  {
  }
  
  #endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */

  #ifdef CONFIG_RCU_BOOST
  
  /*
   * Wake up rcu_kthread() to process callbacks now eligible for invocation
   * or to boost readers.
   */
  static void invoke_rcu_callbacks(void)
  {
  	have_rcu_kthread_work = 1;

  	if (rcu_kthread_task != NULL)
  		wake_up(&rcu_kthread_wq);

  }

  #ifdef CONFIG_RCU_TRACE
  
  /*
   * Is the current CPU running the RCU-callbacks kthread?
   * Caller must have preemption disabled.
   */
  static bool rcu_is_callbacks_kthread(void)
  {
  	return rcu_kthread_task == current;
  }
  
  #endif /* #ifdef CONFIG_RCU_TRACE */

  /*
   * This kthread invokes RCU callbacks whose grace periods have
   * elapsed.  It is awakened as needed, and takes the place of the
   * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
   * This is a kthread, but it is never stopped, at least not until
   * the system goes down.
   */
  static int rcu_kthread(void *arg)
  {
  	unsigned long work;
  	unsigned long morework;
  	unsigned long flags;
  
  	for (;;) {
  		wait_event_interruptible(rcu_kthread_wq,
  					 have_rcu_kthread_work != 0);
  		morework = rcu_boost();
  		local_irq_save(flags);
  		work = have_rcu_kthread_work;
  		have_rcu_kthread_work = morework;
  		local_irq_restore(flags);
  		if (work)
  			rcu_process_callbacks(NULL);
  		schedule_timeout_interruptible(1); /* Leave CPU for others. */
  	}
  
  	return 0;  /* Not reached, but needed to shut gcc up. */
  }
  
  /*
   * Spawn the kthread that invokes RCU callbacks.
   */
  static int __init rcu_spawn_kthreads(void)
  {
  	struct sched_param sp;
  
  	rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
  	sp.sched_priority = RCU_BOOST_PRIO;
  	sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
  	return 0;
  }
  early_initcall(rcu_spawn_kthreads);
  
  #else /* #ifdef CONFIG_RCU_BOOST */

  /* Hold off callback invocation until early_initcall() time. */
  static int rcu_scheduler_fully_active __read_mostly;

  /*
   * Start up softirq processing of callbacks.
   */
  void invoke_rcu_callbacks(void)
  {

  	if (rcu_scheduler_fully_active)
  		raise_softirq(RCU_SOFTIRQ);

  }

  #ifdef CONFIG_RCU_TRACE
  
  /*
   * There is no callback kthread, so this thread is never it.
   */
  static bool rcu_is_callbacks_kthread(void)
  {
  	return false;
  }
  
  #endif /* #ifdef CONFIG_RCU_TRACE */

  static int __init rcu_scheduler_really_started(void)

  {

  	rcu_scheduler_fully_active = 1;

  	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);

  	raise_softirq(RCU_SOFTIRQ);  /* Invoke any callbacks from early boot. */
  	return 0;

  }

  early_initcall(rcu_scheduler_really_started);

  
  #endif /* #else #ifdef CONFIG_RCU_BOOST */

  #ifdef CONFIG_DEBUG_LOCK_ALLOC

  #include <linux/kernel_stat.h>
  
  /*
   * During boot, we forgive RCU lockdep issues.  After this function is
   * invoked, we start taking RCU lockdep issues seriously.
   */

  void __init rcu_scheduler_starting(void)

  {
  	WARN_ON(nr_context_switches() > 0);
  	rcu_scheduler_active = 1;
  }
  
  #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

  #ifdef CONFIG_RCU_TRACE
  
  #ifdef CONFIG_RCU_BOOST
  
  static void rcu_initiate_boost_trace(void)
  {

  	if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
  		rcu_preempt_ctrlblk.n_balk_blkd_tasks++;
  	else if (rcu_preempt_ctrlblk.gp_tasks == NULL &&
  		 rcu_preempt_ctrlblk.exp_tasks == NULL)
  		rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++;

  	else if (rcu_preempt_ctrlblk.boost_tasks != NULL)

  		rcu_preempt_ctrlblk.n_balk_boost_tasks++;

  	else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))

  		rcu_preempt_ctrlblk.n_balk_notyet++;

  	else

  		rcu_preempt_ctrlblk.n_balk_nos++;

  }
  
  #endif /* #ifdef CONFIG_RCU_BOOST */
  
  static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
  {
  	unsigned long flags;

  	local_irq_save(flags);

  	rcp->qlen -= n;

  	local_irq_restore(flags);

  }
  
  /*
   * Dump statistics for TINY_RCU, such as they are.
   */
  static int show_tiny_stats(struct seq_file *m, void *unused)
  {
  	show_tiny_preempt_stats(m);
  	seq_printf(m, "rcu_sched: qlen: %ld
  ", rcu_sched_ctrlblk.qlen);
  	seq_printf(m, "rcu_bh: qlen: %ld
  ", rcu_bh_ctrlblk.qlen);
  	return 0;
  }
  
  static int show_tiny_stats_open(struct inode *inode, struct file *file)
  {
  	return single_open(file, show_tiny_stats, NULL);
  }
  
  static const struct file_operations show_tiny_stats_fops = {
  	.owner = THIS_MODULE,
  	.open = show_tiny_stats_open,
  	.read = seq_read,
  	.llseek = seq_lseek,
  	.release = single_release,
  };
  
  static struct dentry *rcudir;
  
  static int __init rcutiny_trace_init(void)
  {
  	struct dentry *retval;
  
  	rcudir = debugfs_create_dir("rcu", NULL);
  	if (!rcudir)
  		goto free_out;
  	retval = debugfs_create_file("rcudata", 0444, rcudir,
  				     NULL, &show_tiny_stats_fops);
  	if (!retval)
  		goto free_out;
  	return 0;
  free_out:
  	debugfs_remove_recursive(rcudir);
  	return 1;
  }
  
  static void __exit rcutiny_trace_cleanup(void)
  {
  	debugfs_remove_recursive(rcudir);
  }
  
  module_init(rcutiny_trace_init);
  module_exit(rcutiny_trace_cleanup);
  
  MODULE_AUTHOR("Paul E. McKenney");
  MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
  MODULE_LICENSE("GPL");

  static void check_cpu_stall_preempt(void)
  {
  #ifdef CONFIG_TINY_PREEMPT_RCU
  	check_cpu_stall(&rcu_preempt_ctrlblk.rcb);
  #endif /* #ifdef CONFIG_TINY_PREEMPT_RCU */
  }

  #endif /* #ifdef CONFIG_RCU_TRACE */