kernel/rcupdate.c - linux-mtk - Git at Google

 /*
  * 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 <linux/atomic.h>
 #include <linux/bitops.h>
 #include <linux/percpu.h>
 #include <linux/notifier.h>
 #include <linux/cpu.h>
 #include <linux/mutex.h>
 #include <linux/export.h>
 #include <linux/hardirq.h>

 #define CREATE_TRACE_POINTS
 #include <trace/events/rcu.h>

 #include "rcu.h"

 #ifdef CONFIG_PREEMPT_RCU

 /*
  * Check for a task exiting while in a preemptible-RCU read-side
  * critical section, clean up if so.  No need to issue warnings,
  * as debug_check_no_locks_held() already does this if lockdep
  * is enabled.
  */
 void exit_rcu(void)
 {
 	struct task_struct *t = current;

 	if (likely(list_empty(&current->rcu_node_entry)))
 		return;
 	t->rcu_read_lock_nesting = 1;
 	barrier();
 	t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
 	__rcu_read_unlock();
 }

 #else /* #ifdef CONFIG_PREEMPT_RCU */

 void exit_rcu(void)
 {
 }

 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */

 #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);

 static struct lock_class_key rcu_bh_lock_key;
 struct lockdep_map rcu_bh_lock_map =
 	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
 EXPORT_SYMBOL_GPL(rcu_bh_lock_map);

 static struct lock_class_key rcu_sched_lock_key;
 struct lockdep_map rcu_sched_lock_map =
 	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
 #endif

 #ifdef CONFIG_DEBUG_LOCK_ALLOC

 int debug_lockdep_rcu_enabled(void)
 {
 	return rcu_scheduler_active && debug_locks &&
 	       current->lockdep_recursion == 0;
 }
 EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

 /**
  * rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
  *
  * Check for bottom half being disabled, which covers both the
  * CONFIG_PROVE_RCU and not cases.  Note that if someone uses
  * rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
  * will show the situation.  This is useful for debug checks in functions
  * that require that they be called within an RCU read-side critical
  * section.
  *
  * Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
  *
  * Note that rcu_read_lock() is disallowed if the CPU is either idle or
  * offline from an RCU perspective, so check for those as well.
  */
 int rcu_read_lock_bh_held(void)
 {
 	if (!debug_lockdep_rcu_enabled())
 		return 1;
 	if (rcu_is_cpu_idle())
 		return 0;
 	if (!rcu_lockdep_current_cpu_online())
 		return 0;
 	return in_softirq() || irqs_disabled();
 }
 EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);

 #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

 struct rcu_synchronize {
 	struct rcu_head head;
 	struct completion completion;
 };

 /*
  * Awaken the corresponding synchronize_rcu() instance now that a
  * grace period has elapsed.
  */
 static void wakeme_after_rcu(struct rcu_head  *head)
 {
 	struct rcu_synchronize *rcu;

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

 void wait_rcu_gp(call_rcu_func_t crf)
 {
 	struct rcu_synchronize rcu;

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

 #ifdef CONFIG_PROVE_RCU
 /*
  * wrapper function to avoid #include problems.
  */
 int rcu_my_thread_group_empty(void)
 {
 	return thread_group_empty(current);
 }
 EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty);
 #endif /* #ifdef CONFIG_PROVE_RCU */

 #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
 static inline void debug_init_rcu_head(struct rcu_head *head)
 {
 	debug_object_init(head, &rcuhead_debug_descr);
 }

 static inline void debug_rcu_head_free(struct rcu_head *head)
 {
 	debug_object_free(head, &rcuhead_debug_descr);
 }

 /*
  * fixup_init is called when:
  * - an active object is initialized
  */
 static int rcuhead_fixup_init(void *addr, enum debug_obj_state state)
 {
 	struct rcu_head *head = addr;

 	switch (state) {
 	case ODEBUG_STATE_ACTIVE:
 		/*
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
 		 * In !PREEMPT configurations, there is no way to tell if we are
 		 * in a RCU read-side critical section or not, so we never
 		 * attempt any fixup and just print a warning.
 		 */
 #ifndef CONFIG_PREEMPT
 		WARN_ON_ONCE(1);
 		return 0;
 #endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON_ONCE(1);
 			return 0;
 		}
 		rcu_barrier();
 		rcu_barrier_sched();
 		rcu_barrier_bh();
 		debug_object_init(head, &rcuhead_debug_descr);
 		return 1;
 	default:
 		return 0;
 	}
 }

 /*
  * fixup_activate is called when:
  * - an active object is activated
  * - an unknown object is activated (might be a statically initialized object)
  * Activation is performed internally by call_rcu().
  */
 static int rcuhead_fixup_activate(void *addr, enum debug_obj_state state)
 {
 	struct rcu_head *head = addr;

 	switch (state) {

 	case ODEBUG_STATE_NOTAVAILABLE:
 		/*
 		 * This is not really a fixup. We just make sure that it is
 		 * tracked in the object tracker.
 		 */
 		debug_object_init(head, &rcuhead_debug_descr);
 		debug_object_activate(head, &rcuhead_debug_descr);
 		return 0;

 	case ODEBUG_STATE_ACTIVE:
 		/*
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
 		 * In !PREEMPT configurations, there is no way to tell if we are
 		 * in a RCU read-side critical section or not, so we never
 		 * attempt any fixup and just print a warning.
 		 */
 #ifndef CONFIG_PREEMPT
 		WARN_ON_ONCE(1);
 		return 0;
 #endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON_ONCE(1);
 			return 0;
 		}
 		rcu_barrier();
 		rcu_barrier_sched();
 		rcu_barrier_bh();
 		debug_object_activate(head, &rcuhead_debug_descr);
 		return 1;
 	default:
 		return 0;
 	}
 }

 /*
  * fixup_free is called when:
  * - an active object is freed
  */
 static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
 {
 	struct rcu_head *head = addr;

 	switch (state) {
 	case ODEBUG_STATE_ACTIVE:
 		/*
 		 * Ensure that queued callbacks are all executed.
 		 * If we detect that we are nested in a RCU read-side critical
 		 * section, we should simply fail, otherwise we would deadlock.
 		 * In !PREEMPT configurations, there is no way to tell if we are
 		 * in a RCU read-side critical section or not, so we never
 		 * attempt any fixup and just print a warning.
 		 */
 #ifndef CONFIG_PREEMPT
 		WARN_ON_ONCE(1);
 		return 0;
 #endif
 		if (rcu_preempt_depth() != 0 || preempt_count() != 0 ||
 		    irqs_disabled()) {
 			WARN_ON_ONCE(1);
 			return 0;
 		}
 		rcu_barrier();
 		rcu_barrier_sched();
 		rcu_barrier_bh();
 		debug_object_free(head, &rcuhead_debug_descr);
 		return 1;
 	default:
 		return 0;
 	}
 }

 /**
  * init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
  * @head: pointer to rcu_head structure to be initialized
  *
  * This function informs debugobjects of a new rcu_head structure that
  * has been allocated as an auto variable on the stack.  This function
  * is not required for rcu_head structures that are statically defined or
  * that are dynamically allocated on the heap.  This function has no
  * effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
  */
 void init_rcu_head_on_stack(struct rcu_head *head)
 {
 	debug_object_init_on_stack(head, &rcuhead_debug_descr);
 }
 EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);

 /**
  * destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
  * @head: pointer to rcu_head structure to be initialized
  *
  * This function informs debugobjects that an on-stack rcu_head structure
  * is about to go out of scope.  As with init_rcu_head_on_stack(), this
  * function is not required for rcu_head structures that are statically
  * defined or that are dynamically allocated on the heap.  Also as with
  * init_rcu_head_on_stack(), this function has no effect for
  * !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
  */
 void destroy_rcu_head_on_stack(struct rcu_head *head)
 {
 	debug_object_free(head, &rcuhead_debug_descr);
 }
 EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);

 struct debug_obj_descr rcuhead_debug_descr = {
 	.name = "rcu_head",
 	.fixup_init = rcuhead_fixup_init,
 	.fixup_activate = rcuhead_fixup_activate,
 	.fixup_free = rcuhead_fixup_free,
 };
 EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
 #endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */

 #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU) || defined(CONFIG_RCU_TRACE)
 void do_trace_rcu_torture_read(char *rcutorturename, struct rcu_head *rhp)
 {
 	trace_rcu_torture_read(rcutorturename, rhp);
 }
 EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
 #else
 #define do_trace_rcu_torture_read(rcutorturename, rhp) do { } while (0)
 #endif
	/*
	* 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 <linux/atomic.h>
	#include <linux/bitops.h>
	#include <linux/percpu.h>
	#include <linux/notifier.h>
	#include <linux/cpu.h>
	#include <linux/mutex.h>
	#include <linux/export.h>
	#include <linux/hardirq.h>

	#define CREATE_TRACE_POINTS
	#include <trace/events/rcu.h>

	#include "rcu.h"

	#ifdef CONFIG_PREEMPT_RCU

	/*
	* Check for a task exiting while in a preemptible-RCU read-side
	* critical section, clean up if so. No need to issue warnings,
	* as debug_check_no_locks_held() already does this if lockdep
	* is enabled.
	*/
	void exit_rcu(void)
	{
	struct task_struct *t = current;

	if (likely(list_empty(&current->rcu_node_entry)))
	return;
	t->rcu_read_lock_nesting = 1;
	barrier();
	t->rcu_read_unlock_special = RCU_READ_UNLOCK_BLOCKED;
	__rcu_read_unlock();
	}

	#else /* #ifdef CONFIG_PREEMPT_RCU */

	void exit_rcu(void)
	{
	}

	#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

	#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);

	static struct lock_class_key rcu_bh_lock_key;
	struct lockdep_map rcu_bh_lock_map =
	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_bh", &rcu_bh_lock_key);
	EXPORT_SYMBOL_GPL(rcu_bh_lock_map);

	static struct lock_class_key rcu_sched_lock_key;
	struct lockdep_map rcu_sched_lock_map =
	STATIC_LOCKDEP_MAP_INIT("rcu_read_lock_sched", &rcu_sched_lock_key);
	EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
	#endif

	#ifdef CONFIG_DEBUG_LOCK_ALLOC

	int debug_lockdep_rcu_enabled(void)
	{
	return rcu_scheduler_active && debug_locks &&
	current->lockdep_recursion == 0;
	}
	EXPORT_SYMBOL_GPL(debug_lockdep_rcu_enabled);

	/**
	* rcu_read_lock_bh_held() - might we be in RCU-bh read-side critical section?
	*
	* Check for bottom half being disabled, which covers both the
	* CONFIG_PROVE_RCU and not cases. Note that if someone uses
	* rcu_read_lock_bh(), but then later enables BH, lockdep (if enabled)
	* will show the situation. This is useful for debug checks in functions
	* that require that they be called within an RCU read-side critical
	* section.
	*
	* Check debug_lockdep_rcu_enabled() to prevent false positives during boot.
	*
	* Note that rcu_read_lock() is disallowed if the CPU is either idle or
	* offline from an RCU perspective, so check for those as well.
	*/
	int rcu_read_lock_bh_held(void)
	{
	if (!debug_lockdep_rcu_enabled())
	return 1;
	if (rcu_is_cpu_idle())
	return 0;
	if (!rcu_lockdep_current_cpu_online())
	return 0;
	return in_softirq() \|\| irqs_disabled();
	}
	EXPORT_SYMBOL_GPL(rcu_read_lock_bh_held);

	#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

	struct rcu_synchronize {
	struct rcu_head head;
	struct completion completion;
	};

	/*
	* Awaken the corresponding synchronize_rcu() instance now that a
	* grace period has elapsed.
	*/
	static void wakeme_after_rcu(struct rcu_head *head)
	{
	struct rcu_synchronize *rcu;

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

	void wait_rcu_gp(call_rcu_func_t crf)
	{
	struct rcu_synchronize rcu;

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

	#ifdef CONFIG_PROVE_RCU
	/*
	* wrapper function to avoid #include problems.
	*/
	int rcu_my_thread_group_empty(void)
	{
	return thread_group_empty(current);
	}
	EXPORT_SYMBOL_GPL(rcu_my_thread_group_empty);
	#endif /* #ifdef CONFIG_PROVE_RCU */

	#ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD
	static inline void debug_init_rcu_head(struct rcu_head *head)
	{
	debug_object_init(head, &rcuhead_debug_descr);
	}

	static inline void debug_rcu_head_free(struct rcu_head *head)
	{
	debug_object_free(head, &rcuhead_debug_descr);
	}

	/*
	* fixup_init is called when:
	* - an active object is initialized
	*/
	static int rcuhead_fixup_init(void *addr, enum debug_obj_state state)
	{
	struct rcu_head *head = addr;

	switch (state) {
	case ODEBUG_STATE_ACTIVE:
	/*
	* Ensure that queued callbacks are all executed.
	* If we detect that we are nested in a RCU read-side critical
	* section, we should simply fail, otherwise we would deadlock.
	* In !PREEMPT configurations, there is no way to tell if we are
	* in a RCU read-side critical section or not, so we never
	* attempt any fixup and just print a warning.
	*/
	#ifndef CONFIG_PREEMPT
	WARN_ON_ONCE(1);
	return 0;
	#endif
	if (rcu_preempt_depth() != 0 \|\| preempt_count() != 0 \|\|
	irqs_disabled()) {
	WARN_ON_ONCE(1);
	return 0;
	}
	rcu_barrier();
	rcu_barrier_sched();
	rcu_barrier_bh();
	debug_object_init(head, &rcuhead_debug_descr);
	return 1;
	default:
	return 0;
	}
	}

	/*
	* fixup_activate is called when:
	* - an active object is activated
	* - an unknown object is activated (might be a statically initialized object)
	* Activation is performed internally by call_rcu().
	*/
	static int rcuhead_fixup_activate(void *addr, enum debug_obj_state state)
	{
	struct rcu_head *head = addr;

	switch (state) {

	case ODEBUG_STATE_NOTAVAILABLE:
	/*
	* This is not really a fixup. We just make sure that it is
	* tracked in the object tracker.
	*/
	debug_object_init(head, &rcuhead_debug_descr);
	debug_object_activate(head, &rcuhead_debug_descr);
	return 0;

	case ODEBUG_STATE_ACTIVE:
	/*
	* Ensure that queued callbacks are all executed.
	* If we detect that we are nested in a RCU read-side critical
	* section, we should simply fail, otherwise we would deadlock.
	* In !PREEMPT configurations, there is no way to tell if we are
	* in a RCU read-side critical section or not, so we never
	* attempt any fixup and just print a warning.
	*/
	#ifndef CONFIG_PREEMPT
	WARN_ON_ONCE(1);
	return 0;
	#endif
	if (rcu_preempt_depth() != 0 \|\| preempt_count() != 0 \|\|
	irqs_disabled()) {
	WARN_ON_ONCE(1);
	return 0;
	}
	rcu_barrier();
	rcu_barrier_sched();
	rcu_barrier_bh();
	debug_object_activate(head, &rcuhead_debug_descr);
	return 1;
	default:
	return 0;
	}
	}

	/*
	* fixup_free is called when:
	* - an active object is freed
	*/
	static int rcuhead_fixup_free(void *addr, enum debug_obj_state state)
	{
	struct rcu_head *head = addr;

	switch (state) {
	case ODEBUG_STATE_ACTIVE:
	/*
	* Ensure that queued callbacks are all executed.
	* If we detect that we are nested in a RCU read-side critical
	* section, we should simply fail, otherwise we would deadlock.
	* In !PREEMPT configurations, there is no way to tell if we are
	* in a RCU read-side critical section or not, so we never
	* attempt any fixup and just print a warning.
	*/
	#ifndef CONFIG_PREEMPT
	WARN_ON_ONCE(1);
	return 0;
	#endif
	if (rcu_preempt_depth() != 0 \|\| preempt_count() != 0 \|\|
	irqs_disabled()) {
	WARN_ON_ONCE(1);
	return 0;
	}
	rcu_barrier();
	rcu_barrier_sched();
	rcu_barrier_bh();
	debug_object_free(head, &rcuhead_debug_descr);
	return 1;
	default:
	return 0;
	}
	}

	/**
	* init_rcu_head_on_stack() - initialize on-stack rcu_head for debugobjects
	* @head: pointer to rcu_head structure to be initialized
	*
	* This function informs debugobjects of a new rcu_head structure that
	* has been allocated as an auto variable on the stack. This function
	* is not required for rcu_head structures that are statically defined or
	* that are dynamically allocated on the heap. This function has no
	* effect for !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
	*/
	void init_rcu_head_on_stack(struct rcu_head *head)
	{
	debug_object_init_on_stack(head, &rcuhead_debug_descr);
	}
	EXPORT_SYMBOL_GPL(init_rcu_head_on_stack);

	/**
	* destroy_rcu_head_on_stack() - destroy on-stack rcu_head for debugobjects
	* @head: pointer to rcu_head structure to be initialized
	*
	* This function informs debugobjects that an on-stack rcu_head structure
	* is about to go out of scope. As with init_rcu_head_on_stack(), this
	* function is not required for rcu_head structures that are statically
	* defined or that are dynamically allocated on the heap. Also as with
	* init_rcu_head_on_stack(), this function has no effect for
	* !CONFIG_DEBUG_OBJECTS_RCU_HEAD kernel builds.
	*/
	void destroy_rcu_head_on_stack(struct rcu_head *head)
	{
	debug_object_free(head, &rcuhead_debug_descr);
	}
	EXPORT_SYMBOL_GPL(destroy_rcu_head_on_stack);

	struct debug_obj_descr rcuhead_debug_descr = {
	.name = "rcu_head",
	.fixup_init = rcuhead_fixup_init,
	.fixup_activate = rcuhead_fixup_activate,
	.fixup_free = rcuhead_fixup_free,
	};
	EXPORT_SYMBOL_GPL(rcuhead_debug_descr);
	#endif /* #ifdef CONFIG_DEBUG_OBJECTS_RCU_HEAD */

	#if defined(CONFIG_TREE_RCU) \|\| defined(CONFIG_TREE_PREEMPT_RCU) \|\| defined(CONFIG_RCU_TRACE)
	void do_trace_rcu_torture_read(char rcutorturename, struct rcu_head rhp)
	{
	trace_rcu_torture_read(rcutorturename, rhp);
	}
	EXPORT_SYMBOL_GPL(do_trace_rcu_torture_read);
	#else
	#define do_trace_rcu_torture_read(rcutorturename, rhp) do { } while (0)
	#endif