kernel/rcuclassic.c - kernel/msm-4.9 - Gitiles

 /*
  * 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 -
  * 		Documentation/RCU
  *
  */
 #include <linux/types.h>
 #include <linux/kernel.h>
 #include <linux/init.h>
 #include <linux/spinlock.h>
 #include <linux/smp.h>
 #include <linux/rcupdate.h>
 #include <linux/interrupt.h>
 #include <linux/sched.h>
 #include <asm/atomic.h>
 #include <linux/bitops.h>
 #include <linux/module.h>
 #include <linux/completion.h>
 #include <linux/moduleparam.h>
 #include <linux/percpu.h>
 #include <linux/notifier.h>
 #include <linux/cpu.h>
 #include <linux/mutex.h>

 #ifdef CONFIG_DEBUG_LOCK_ALLOC
 static struct lock_class_key rcu_lock_key;
 struct lockdep_map rcu_lock_map =
 	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
 EXPORT_SYMBOL_GPL(rcu_lock_map);
 #endif


 /* Definition for rcupdate control block. */
 static struct rcu_ctrlblk rcu_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
 	.cpumask = CPU_MASK_NONE,
 };
 static struct rcu_ctrlblk rcu_bh_ctrlblk = {
 	.cur = -300,
 	.completed = -300,
 	.lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
 	.cpumask = CPU_MASK_NONE,
 };

 DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
 DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };

 static int blimit = 10;
 static int qhimark = 10000;
 static int qlowmark = 100;

 #ifdef CONFIG_SMP
 static void force_quiescent_state(struct rcu_data *rdp,
 			struct rcu_ctrlblk *rcp)
 {
 	int cpu;
 	cpumask_t cpumask;
 	set_need_resched();
 	if (unlikely(!rcp->signaled)) {
 		rcp->signaled = 1;
 		/*
 		 * Don't send IPI to itself. With irqs disabled,
 		 * rdp->cpu is the current cpu.
 		 *
 		 * cpu_online_map is updated by the _cpu_down()
 		 * using stop_machine_run(). Since we're in irqs disabled
 		 * section, stop_machine_run() is not exectuting, hence
 		 * the cpu_online_map is stable.
 		 *
 		 * However,  a cpu might have been offlined _just_ before
 		 * we disabled irqs while entering here.
 		 * And rcu subsystem might not yet have handled the CPU_DEAD
 		 * notification, leading to the offlined cpu's bit
 		 * being set in the rcp->cpumask.
 		 *
 		 * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
 		 * sending smp_reschedule() to an offlined CPU.
 		 */
 		cpus_and(cpumask, rcp->cpumask, cpu_online_map);
 		cpu_clear(rdp->cpu, cpumask);
 		for_each_cpu_mask(cpu, cpumask)
 			smp_send_reschedule(cpu);
 	}
 }
 #else
 static inline void force_quiescent_state(struct rcu_data *rdp,
 			struct rcu_ctrlblk *rcp)
 {
 	set_need_resched();
 }
 #endif

 /**
  * call_rcu - Queue an RCU callback for invocation after a grace period.
  * @head: structure to be used for queueing the RCU updates.
  * @func: actual update function to be invoked after the grace period
  *
  * The update function will be invoked some time after a full grace
  * period elapses, 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 call_rcu(struct rcu_head *head,
 				void (*func)(struct rcu_head *rcu))
 {
 	unsigned long flags;
 	struct rcu_data *rdp;

 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
 	rdp = &__get_cpu_var(rcu_data);
 	*rdp->nxttail = head;
 	rdp->nxttail = &head->next;
 	if (unlikely(++rdp->qlen > qhimark)) {
 		rdp->blimit = INT_MAX;
 		force_quiescent_state(rdp, &rcu_ctrlblk);
 	}
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(call_rcu);

 /**
  * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
  * @head: structure to be used for queueing the RCU updates.
  * @func: actual update function to be invoked after the grace period
  *
  * The update function will be invoked some time after a full grace
  * period elapses, in other words after all currently executing RCU
  * read-side critical sections have completed. call_rcu_bh() assumes
  * that the read-side critical sections end on completion of a softirq
  * handler. This means that read-side critical sections in process
  * context must not be interrupted by softirqs. This interface is to be
  * used when most of the read-side critical sections are in softirq context.
  * RCU read-side critical sections are delimited by rcu_read_lock() and
  * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
  * and rcu_read_unlock_bh(), if in process context. These may be nested.
  */
 void call_rcu_bh(struct rcu_head *head,
 				void (*func)(struct rcu_head *rcu))
 {
 	unsigned long flags;
 	struct rcu_data *rdp;

 	head->func = func;
 	head->next = NULL;
 	local_irq_save(flags);
 	rdp = &__get_cpu_var(rcu_bh_data);
 	*rdp->nxttail = head;
 	rdp->nxttail = &head->next;

 	if (unlikely(++rdp->qlen > qhimark)) {
 		rdp->blimit = INT_MAX;
 		force_quiescent_state(rdp, &rcu_bh_ctrlblk);
 	}

 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(call_rcu_bh);

 /*
  * Return the number of RCU batches processed thus far.  Useful
  * for debug and statistics.
  */
 long rcu_batches_completed(void)
 {
 	return rcu_ctrlblk.completed;
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed);

 /*
  * Return the number of RCU batches processed thus far.  Useful
  * for debug and statistics.
  */
 long rcu_batches_completed_bh(void)
 {
 	return rcu_bh_ctrlblk.completed;
 }
 EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);

 /* Raises the softirq for processing rcu_callbacks. */
 static inline void raise_rcu_softirq(void)
 {
 	raise_softirq(RCU_SOFTIRQ);
 	/*
 	 * The smp_mb() here is required to ensure that this cpu's
 	 * __rcu_process_callbacks() reads the most recently updated
 	 * value of rcu->cur.
 	 */
 	smp_mb();
 }

 /*
  * Invoke the completed RCU callbacks. They are expected to be in
  * a per-cpu list.
  */
 static void rcu_do_batch(struct rcu_data *rdp)
 {
 	struct rcu_head *next, *list;
 	int count = 0;

 	list = rdp->donelist;
 	while (list) {
 		next = list->next;
 		prefetch(next);
 		list->func(list);
 		list = next;
 		if (++count >= rdp->blimit)
 			break;
 	}
 	rdp->donelist = list;

 	local_irq_disable();
 	rdp->qlen -= count;
 	local_irq_enable();
 	if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
 		rdp->blimit = blimit;

 	if (!rdp->donelist)
 		rdp->donetail = &rdp->donelist;
 	else
 		raise_rcu_softirq();
 }

 /*
  * Grace period handling:
  * The grace period handling consists out of two steps:
  * - A new grace period is started.
  *   This is done by rcu_start_batch. The start is not broadcasted to
  *   all cpus, they must pick this up by comparing rcp->cur with
  *   rdp->quiescbatch. All cpus are recorded  in the
  *   rcu_ctrlblk.cpumask bitmap.
  * - All cpus must go through a quiescent state.
  *   Since the start of the grace period is not broadcasted, at least two
  *   calls to rcu_check_quiescent_state are required:
  *   The first call just notices that a new grace period is running. The
  *   following calls check if there was a quiescent state since the beginning
  *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
  *   the bitmap is empty, then the grace period is completed.
  *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
  *   period (if necessary).
  */
 /*
  * Register a new batch of callbacks, and start it up if there is currently no
  * active batch and the batch to be registered has not already occurred.
  * Caller must hold rcu_ctrlblk.lock.
  */
 static void rcu_start_batch(struct rcu_ctrlblk *rcp)
 {
 	if (rcp->next_pending &&
 			rcp->completed == rcp->cur) {
 		rcp->next_pending = 0;
 		/*
 		 * next_pending == 0 must be visible in
 		 * __rcu_process_callbacks() before it can see new value of cur.
 		 */
 		smp_wmb();
 		rcp->cur++;

 		/*
 		 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
 		 * Barrier  Otherwise it can cause tickless idle CPUs to be
 		 * included in rcp->cpumask, which will extend graceperiods
 		 * unnecessarily.
 		 */
 		smp_mb();
 		cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);

 		rcp->signaled = 0;
 	}
 }

 /*
  * cpu went through a quiescent state since the beginning of the grace period.
  * Clear it from the cpu mask and complete the grace period if it was the last
  * cpu. Start another grace period if someone has further entries pending
  */
 static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
 {
 	cpu_clear(cpu, rcp->cpumask);
 	if (cpus_empty(rcp->cpumask)) {
 		/* batch completed ! */
 		rcp->completed = rcp->cur;
 		rcu_start_batch(rcp);
 	}
 }

 /*
  * Check if the cpu has gone through a quiescent state (say context
  * switch). If so and if it already hasn't done so in this RCU
  * quiescent cycle, then indicate that it has done so.
  */
 static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
 					struct rcu_data *rdp)
 {
 	if (rdp->quiescbatch != rcp->cur) {
 		/* start new grace period: */
 		rdp->qs_pending = 1;
 		rdp->passed_quiesc = 0;
 		rdp->quiescbatch = rcp->cur;
 		return;
 	}

 	/* Grace period already completed for this cpu?
 	 * qs_pending is checked instead of the actual bitmap to avoid
 	 * cacheline trashing.
 	 */
 	if (!rdp->qs_pending)
 		return;

 	/*
 	 * Was there a quiescent state since the beginning of the grace
 	 * period? If no, then exit and wait for the next call.
 	 */
 	if (!rdp->passed_quiesc)
 		return;
 	rdp->qs_pending = 0;

 	spin_lock(&rcp->lock);
 	/*
 	 * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
 	 * during cpu startup. Ignore the quiescent state.
 	 */
 	if (likely(rdp->quiescbatch == rcp->cur))
 		cpu_quiet(rdp->cpu, rcp);

 	spin_unlock(&rcp->lock);
 }


 #ifdef CONFIG_HOTPLUG_CPU

 /* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
  * locking requirements, the list it's pulling from has to belong to a cpu
  * which is dead and hence not processing interrupts.
  */
 static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
 				struct rcu_head **tail)
 {
 	local_irq_disable();
 	*this_rdp->nxttail = list;
 	if (list)
 		this_rdp->nxttail = tail;
 	local_irq_enable();
 }

 static void __rcu_offline_cpu(struct rcu_data *this_rdp,
 				struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 {
 	/* if the cpu going offline owns the grace period
 	 * we can block indefinitely waiting for it, so flush
 	 * it here
 	 */
 	spin_lock_bh(&rcp->lock);
 	if (rcp->cur != rcp->completed)
 		cpu_quiet(rdp->cpu, rcp);
 	spin_unlock_bh(&rcp->lock);
 	rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
 	rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
 	rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
 }

 static void rcu_offline_cpu(int cpu)
 {
 	struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
 	struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);

 	__rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
 					&per_cpu(rcu_data, cpu));
 	__rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
 					&per_cpu(rcu_bh_data, cpu));
 	put_cpu_var(rcu_data);
 	put_cpu_var(rcu_bh_data);
 }

 #else

 static void rcu_offline_cpu(int cpu)
 {
 }

 #endif

 /*
  * This does the RCU processing work from softirq context.
  */
 static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
 					struct rcu_data *rdp)
 {
 	if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
 		*rdp->donetail = rdp->curlist;
 		rdp->donetail = rdp->curtail;
 		rdp->curlist = NULL;
 		rdp->curtail = &rdp->curlist;
 	}

 	if (rdp->nxtlist && !rdp->curlist) {
 		local_irq_disable();
 		rdp->curlist = rdp->nxtlist;
 		rdp->curtail = rdp->nxttail;
 		rdp->nxtlist = NULL;
 		rdp->nxttail = &rdp->nxtlist;
 		local_irq_enable();

 		/*
 		 * start the next batch of callbacks
 		 */

 		/* determine batch number */
 		rdp->batch = rcp->cur + 1;
 		/* see the comment and corresponding wmb() in
 		 * the rcu_start_batch()
 		 */
 		smp_rmb();

 		if (!rcp->next_pending) {
 			/* and start it/schedule start if it's a new batch */
 			spin_lock(&rcp->lock);
 			rcp->next_pending = 1;
 			rcu_start_batch(rcp);
 			spin_unlock(&rcp->lock);
 		}
 	}

 	rcu_check_quiescent_state(rcp, rdp);
 	if (rdp->donelist)
 		rcu_do_batch(rdp);
 }

 static void rcu_process_callbacks(struct softirq_action *unused)
 {
 	__rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
 	__rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
 }

 static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
 {
 	/* This cpu has pending rcu entries and the grace period
 	 * for them has completed.
 	 */
 	if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
 		return 1;

 	/* This cpu has no pending entries, but there are new entries */
 	if (!rdp->curlist && rdp->nxtlist)
 		return 1;

 	/* This cpu has finished callbacks to invoke */
 	if (rdp->donelist)
 		return 1;

 	/* The rcu core waits for a quiescent state from the cpu */
 	if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
 		return 1;

 	/* nothing to do */
 	return 0;
 }

 /*
  * Check to see if there is any immediate RCU-related work to be done
  * by the current CPU, returning 1 if so.  This function is part of the
  * RCU implementation; it is -not- an exported member of the RCU API.
  */
 int rcu_pending(int cpu)
 {
 	return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
 		__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
 }

 /*
  * Check to see if any future RCU-related work will need to be done
  * by the current CPU, even if none need be done immediately, returning
  * 1 if so.  This function is part of the RCU implementation; it is -not-
  * an exported member of the RCU API.
  */
 int rcu_needs_cpu(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);

 	return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
 }

 void rcu_check_callbacks(int cpu, int user)
 {
 	if (user ||
 	    (idle_cpu(cpu) && !in_softirq() &&
 				hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
 		rcu_qsctr_inc(cpu);
 		rcu_bh_qsctr_inc(cpu);
 	} else if (!in_softirq())
 		rcu_bh_qsctr_inc(cpu);
 	raise_rcu_softirq();
 }

 static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
 						struct rcu_data *rdp)
 {
 	memset(rdp, 0, sizeof(*rdp));
 	rdp->curtail = &rdp->curlist;
 	rdp->nxttail = &rdp->nxtlist;
 	rdp->donetail = &rdp->donelist;
 	rdp->quiescbatch = rcp->completed;
 	rdp->qs_pending = 0;
 	rdp->cpu = cpu;
 	rdp->blimit = blimit;
 }

 static void __cpuinit rcu_online_cpu(int cpu)
 {
 	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
 	struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);

 	rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
 	rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
 	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
 }

 static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
 				unsigned long action, void *hcpu)
 {
 	long cpu = (long)hcpu;

 	switch (action) {
 	case CPU_UP_PREPARE:
 	case CPU_UP_PREPARE_FROZEN:
 		rcu_online_cpu(cpu);
 		break;
 	case CPU_DEAD:
 	case CPU_DEAD_FROZEN:
 		rcu_offline_cpu(cpu);
 		break;
 	default:
 		break;
 	}
 	return NOTIFY_OK;
 }

 static struct notifier_block __cpuinitdata rcu_nb = {
 	.notifier_call	= rcu_cpu_notify,
 };

 /*
  * Initializes rcu mechanism.  Assumed to be called early.
  * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
  * Note that rcu_qsctr and friends are implicitly
  * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
  */
 void __init __rcu_init(void)
 {
 	rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
 			(void *)(long)smp_processor_id());
 	/* Register notifier for non-boot CPUs */
 	register_cpu_notifier(&rcu_nb);
 }

 module_param(blimit, int, 0);
 module_param(qhimark, int, 0);
 module_param(qlowmark, int, 0);
	/*
	* 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 -
	* Documentation/RCU
	*
	*/
	#include <linux/types.h>
	#include <linux/kernel.h>
	#include <linux/init.h>
	#include <linux/spinlock.h>
	#include <linux/smp.h>
	#include <linux/rcupdate.h>
	#include <linux/interrupt.h>
	#include <linux/sched.h>
	#include <asm/atomic.h>
	#include <linux/bitops.h>
	#include <linux/module.h>
	#include <linux/completion.h>
	#include <linux/moduleparam.h>
	#include <linux/percpu.h>
	#include <linux/notifier.h>
	#include <linux/cpu.h>
	#include <linux/mutex.h>

	#ifdef CONFIG_DEBUG_LOCK_ALLOC
	static struct lock_class_key rcu_lock_key;
	struct lockdep_map rcu_lock_map =
	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
	EXPORT_SYMBOL_GPL(rcu_lock_map);
	#endif


	/* Definition for rcupdate control block. */
	static struct rcu_ctrlblk rcu_ctrlblk = {
	.cur = -300,
	.completed = -300,
	.lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
	.cpumask = CPU_MASK_NONE,
	};
	static struct rcu_ctrlblk rcu_bh_ctrlblk = {
	.cur = -300,
	.completed = -300,
	.lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
	.cpumask = CPU_MASK_NONE,
	};

	DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
	DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };

	static int blimit = 10;
	static int qhimark = 10000;
	static int qlowmark = 100;

	#ifdef CONFIG_SMP
	static void force_quiescent_state(struct rcu_data *rdp,
	struct rcu_ctrlblk *rcp)
	{
	int cpu;
	cpumask_t cpumask;
	set_need_resched();
	if (unlikely(!rcp->signaled)) {
	rcp->signaled = 1;
	/*
	* Don't send IPI to itself. With irqs disabled,
	* rdp->cpu is the current cpu.
	*
	* cpu_online_map is updated by the _cpu_down()
	* using stop_machine_run(). Since we're in irqs disabled
	* section, stop_machine_run() is not exectuting, hence
	* the cpu_online_map is stable.
	*
	* However, a cpu might have been offlined _just_ before
	* we disabled irqs while entering here.
	* And rcu subsystem might not yet have handled the CPU_DEAD
	* notification, leading to the offlined cpu's bit
	* being set in the rcp->cpumask.
	*
	* Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
	* sending smp_reschedule() to an offlined CPU.
	*/
	cpus_and(cpumask, rcp->cpumask, cpu_online_map);
	cpu_clear(rdp->cpu, cpumask);
	for_each_cpu_mask(cpu, cpumask)
	smp_send_reschedule(cpu);
	}
	}
	#else
	static inline void force_quiescent_state(struct rcu_data *rdp,
	struct rcu_ctrlblk *rcp)
	{
	set_need_resched();
	}
	#endif

	/**
	* call_rcu - Queue an RCU callback for invocation after a grace period.
	* @head: structure to be used for queueing the RCU updates.
	* @func: actual update function to be invoked after the grace period
	*
	* The update function will be invoked some time after a full grace
	* period elapses, 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 call_rcu(struct rcu_head *head,
	void (func)(struct rcu_head rcu))
	{
	unsigned long flags;
	struct rcu_data *rdp;

	head->func = func;
	head->next = NULL;
	local_irq_save(flags);
	rdp = &__get_cpu_var(rcu_data);
	*rdp->nxttail = head;
	rdp->nxttail = &head->next;
	if (unlikely(++rdp->qlen > qhimark)) {
	rdp->blimit = INT_MAX;
	force_quiescent_state(rdp, &rcu_ctrlblk);
	}
	local_irq_restore(flags);
	}
	EXPORT_SYMBOL_GPL(call_rcu);

	/**
	* call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
	* @head: structure to be used for queueing the RCU updates.
	* @func: actual update function to be invoked after the grace period
	*
	* The update function will be invoked some time after a full grace
	* period elapses, in other words after all currently executing RCU
	* read-side critical sections have completed. call_rcu_bh() assumes
	* that the read-side critical sections end on completion of a softirq
	* handler. This means that read-side critical sections in process
	* context must not be interrupted by softirqs. This interface is to be
	* used when most of the read-side critical sections are in softirq context.
	* RCU read-side critical sections are delimited by rcu_read_lock() and
	* rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
	* and rcu_read_unlock_bh(), if in process context. These may be nested.
	*/
	void call_rcu_bh(struct rcu_head *head,
	void (func)(struct rcu_head rcu))
	{
	unsigned long flags;
	struct rcu_data *rdp;

	head->func = func;
	head->next = NULL;
	local_irq_save(flags);
	rdp = &__get_cpu_var(rcu_bh_data);
	*rdp->nxttail = head;
	rdp->nxttail = &head->next;

	if (unlikely(++rdp->qlen > qhimark)) {
	rdp->blimit = INT_MAX;
	force_quiescent_state(rdp, &rcu_bh_ctrlblk);
	}

	local_irq_restore(flags);
	}
	EXPORT_SYMBOL_GPL(call_rcu_bh);

	/*
	* Return the number of RCU batches processed thus far. Useful
	* for debug and statistics.
	*/
	long rcu_batches_completed(void)
	{
	return rcu_ctrlblk.completed;
	}
	EXPORT_SYMBOL_GPL(rcu_batches_completed);

	/*
	* Return the number of RCU batches processed thus far. Useful
	* for debug and statistics.
	*/
	long rcu_batches_completed_bh(void)
	{
	return rcu_bh_ctrlblk.completed;
	}
	EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);

	/* Raises the softirq for processing rcu_callbacks. */
	static inline void raise_rcu_softirq(void)
	{
	raise_softirq(RCU_SOFTIRQ);
	/*
	* The smp_mb() here is required to ensure that this cpu's
	* __rcu_process_callbacks() reads the most recently updated
	* value of rcu->cur.
	*/
	smp_mb();
	}

	/*
	* Invoke the completed RCU callbacks. They are expected to be in
	* a per-cpu list.
	*/
	static void rcu_do_batch(struct rcu_data *rdp)
	{
	struct rcu_head next, list;
	int count = 0;

	list = rdp->donelist;
	while (list) {
	next = list->next;
	prefetch(next);
	list->func(list);
	list = next;
	if (++count >= rdp->blimit)
	break;
	}
	rdp->donelist = list;

	local_irq_disable();
	rdp->qlen -= count;
	local_irq_enable();
	if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
	rdp->blimit = blimit;

	if (!rdp->donelist)
	rdp->donetail = &rdp->donelist;
	else
	raise_rcu_softirq();
	}

	/*
	* Grace period handling:
	* The grace period handling consists out of two steps:
	* - A new grace period is started.
	* This is done by rcu_start_batch. The start is not broadcasted to
	* all cpus, they must pick this up by comparing rcp->cur with
	* rdp->quiescbatch. All cpus are recorded in the
	* rcu_ctrlblk.cpumask bitmap.
	* - All cpus must go through a quiescent state.
	* Since the start of the grace period is not broadcasted, at least two
	* calls to rcu_check_quiescent_state are required:
	* The first call just notices that a new grace period is running. The
	* following calls check if there was a quiescent state since the beginning
	* of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
	* the bitmap is empty, then the grace period is completed.
	* rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
	* period (if necessary).
	*/
	/*
	* Register a new batch of callbacks, and start it up if there is currently no
	* active batch and the batch to be registered has not already occurred.
	* Caller must hold rcu_ctrlblk.lock.
	*/
	static void rcu_start_batch(struct rcu_ctrlblk *rcp)
	{
	if (rcp->next_pending &&
	rcp->completed == rcp->cur) {
	rcp->next_pending = 0;
	/*
	* next_pending == 0 must be visible in
	* __rcu_process_callbacks() before it can see new value of cur.
	*/
	smp_wmb();
	rcp->cur++;

	/*
	* Accessing nohz_cpu_mask before incrementing rcp->cur needs a
	* Barrier Otherwise it can cause tickless idle CPUs to be
	* included in rcp->cpumask, which will extend graceperiods
	* unnecessarily.
	*/
	smp_mb();
	cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);

	rcp->signaled = 0;
	}
	}

	/*
	* cpu went through a quiescent state since the beginning of the grace period.
	* Clear it from the cpu mask and complete the grace period if it was the last
	* cpu. Start another grace period if someone has further entries pending
	*/
	static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
	{
	cpu_clear(cpu, rcp->cpumask);
	if (cpus_empty(rcp->cpumask)) {
	/* batch completed ! */
	rcp->completed = rcp->cur;
	rcu_start_batch(rcp);
	}
	}

	/*
	* Check if the cpu has gone through a quiescent state (say context
	* switch). If so and if it already hasn't done so in this RCU
	* quiescent cycle, then indicate that it has done so.
	*/
	static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
	struct rcu_data *rdp)
	{
	if (rdp->quiescbatch != rcp->cur) {
	/* start new grace period: */
	rdp->qs_pending = 1;
	rdp->passed_quiesc = 0;
	rdp->quiescbatch = rcp->cur;
	return;
	}

	/* Grace period already completed for this cpu?
	* qs_pending is checked instead of the actual bitmap to avoid
	* cacheline trashing.
	*/
	if (!rdp->qs_pending)
	return;

	/*
	* Was there a quiescent state since the beginning of the grace
	* period? If no, then exit and wait for the next call.
	*/
	if (!rdp->passed_quiesc)
	return;
	rdp->qs_pending = 0;

	spin_lock(&rcp->lock);
	/*
	* rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
	* during cpu startup. Ignore the quiescent state.
	*/
	if (likely(rdp->quiescbatch == rcp->cur))
	cpu_quiet(rdp->cpu, rcp);

	spin_unlock(&rcp->lock);
	}


	#ifdef CONFIG_HOTPLUG_CPU

	/* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
	* locking requirements, the list it's pulling from has to belong to a cpu
	* which is dead and hence not processing interrupts.
	*/
	static void rcu_move_batch(struct rcu_data this_rdp, struct rcu_head list,
	struct rcu_head **tail)
	{
	local_irq_disable();
	*this_rdp->nxttail = list;
	if (list)
	this_rdp->nxttail = tail;
	local_irq_enable();
	}

	static void __rcu_offline_cpu(struct rcu_data *this_rdp,
	struct rcu_ctrlblk rcp, struct rcu_data rdp)
	{
	/* if the cpu going offline owns the grace period
	* we can block indefinitely waiting for it, so flush
	* it here
	*/
	spin_lock_bh(&rcp->lock);
	if (rcp->cur != rcp->completed)
	cpu_quiet(rdp->cpu, rcp);
	spin_unlock_bh(&rcp->lock);
	rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail);
	rcu_move_batch(this_rdp, rdp->curlist, rdp->curtail);
	rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail);
	}

	static void rcu_offline_cpu(int cpu)
	{
	struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
	struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);

	__rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
	&per_cpu(rcu_data, cpu));
	__rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
	&per_cpu(rcu_bh_data, cpu));
	put_cpu_var(rcu_data);
	put_cpu_var(rcu_bh_data);
	}

	#else

	static void rcu_offline_cpu(int cpu)
	{
	}

	#endif

	/*
	* This does the RCU processing work from softirq context.
	*/
	static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
	struct rcu_data *rdp)
	{
	if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch)) {
	*rdp->donetail = rdp->curlist;
	rdp->donetail = rdp->curtail;
	rdp->curlist = NULL;
	rdp->curtail = &rdp->curlist;
	}

	if (rdp->nxtlist && !rdp->curlist) {
	local_irq_disable();
	rdp->curlist = rdp->nxtlist;
	rdp->curtail = rdp->nxttail;
	rdp->nxtlist = NULL;
	rdp->nxttail = &rdp->nxtlist;
	local_irq_enable();

	/*
	* start the next batch of callbacks
	*/

	/* determine batch number */
	rdp->batch = rcp->cur + 1;
	/* see the comment and corresponding wmb() in
	* the rcu_start_batch()
	*/
	smp_rmb();

	if (!rcp->next_pending) {
	/* and start it/schedule start if it's a new batch */
	spin_lock(&rcp->lock);
	rcp->next_pending = 1;
	rcu_start_batch(rcp);
	spin_unlock(&rcp->lock);
	}
	}

	rcu_check_quiescent_state(rcp, rdp);
	if (rdp->donelist)
	rcu_do_batch(rdp);
	}

	static void rcu_process_callbacks(struct softirq_action *unused)
	{
	__rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
	__rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
	}

	static int __rcu_pending(struct rcu_ctrlblk rcp, struct rcu_data rdp)
	{
	/* This cpu has pending rcu entries and the grace period
	* for them has completed.
	*/
	if (rdp->curlist && !rcu_batch_before(rcp->completed, rdp->batch))
	return 1;

	/* This cpu has no pending entries, but there are new entries */
	if (!rdp->curlist && rdp->nxtlist)
	return 1;

	/* This cpu has finished callbacks to invoke */
	if (rdp->donelist)
	return 1;

	/* The rcu core waits for a quiescent state from the cpu */
	if (rdp->quiescbatch != rcp->cur \|\| rdp->qs_pending)
	return 1;

	/* nothing to do */
	return 0;
	}

	/*
	* Check to see if there is any immediate RCU-related work to be done
	* by the current CPU, returning 1 if so. This function is part of the
	* RCU implementation; it is -not- an exported member of the RCU API.
	*/
	int rcu_pending(int cpu)
	{
	return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) \|\|
	__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
	}

	/*
	* Check to see if any future RCU-related work will need to be done
	* by the current CPU, even if none need be done immediately, returning
	* 1 if so. This function is part of the RCU implementation; it is -not-
	* an exported member of the RCU API.
	*/
	int rcu_needs_cpu(int cpu)
	{
	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);

	return (!!rdp->curlist \|\| !!rdp_bh->curlist \|\| rcu_pending(cpu));
	}

	void rcu_check_callbacks(int cpu, int user)
	{
	if (user \|\|
	(idle_cpu(cpu) && !in_softirq() &&
	hardirq_count() <= (1 << HARDIRQ_SHIFT))) {
	rcu_qsctr_inc(cpu);
	rcu_bh_qsctr_inc(cpu);
	} else if (!in_softirq())
	rcu_bh_qsctr_inc(cpu);
	raise_rcu_softirq();
	}

	static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
	struct rcu_data *rdp)
	{
	memset(rdp, 0, sizeof(*rdp));
	rdp->curtail = &rdp->curlist;
	rdp->nxttail = &rdp->nxtlist;
	rdp->donetail = &rdp->donelist;
	rdp->quiescbatch = rcp->completed;
	rdp->qs_pending = 0;
	rdp->cpu = cpu;
	rdp->blimit = blimit;
	}

	static void __cpuinit rcu_online_cpu(int cpu)
	{
	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
	struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);

	rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
	rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
	}

	static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
	unsigned long action, void *hcpu)
	{
	long cpu = (long)hcpu;

	switch (action) {
	case CPU_UP_PREPARE:
	case CPU_UP_PREPARE_FROZEN:
	rcu_online_cpu(cpu);
	break;
	case CPU_DEAD:
	case CPU_DEAD_FROZEN:
	rcu_offline_cpu(cpu);
	break;
	default:
	break;
	}
	return NOTIFY_OK;
	}

	static struct notifier_block __cpuinitdata rcu_nb = {
	.notifier_call = rcu_cpu_notify,
	};

	/*
	* Initializes rcu mechanism. Assumed to be called early.
	* That is before local timer(SMP) or jiffie timer (uniproc) is setup.
	* Note that rcu_qsctr and friends are implicitly
	* initialized due to the choice of ``0'' for RCU_CTR_INVALID.
	*/
	void __init __rcu_init(void)
	{
	rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
	(void *)(long)smp_processor_id());
	/* Register notifier for non-boot CPUs */
	register_cpu_notifier(&rcu_nb);
	}

	module_param(blimit, int, 0);
	module_param(qhimark, int, 0);
	module_param(qlowmark, int, 0);