Date:	Mon, 12 Oct 2009 17:29:25 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, dvhltc@...ibm.com, niv@...ibm.com,
	tglx@...utronix.de, peterz@...radead.org, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com, avi@...hat.com,
	mtosatti@...hat.com, torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC tip/core/rcu 1/3] rcu: The Bloatwatch Edition, v7



Paul E. McKenney wrote:
> This patch is a version of RCU designed for !SMP provided for a
> small-footprint RCU implementation.  In particular, the implementation
> of synchronize_rcu() is extremely lightweight and high performance.
> It passes rcutorture testing in each of the four relevant configurations
> (combinations of NO_HZ and PREEMPT) on x86.  This saves about 1K bytes
> compared to old Classic RCU (which is no longer in mainline), and more
> than three kilobytes compared to Hierarchical RCU (updated to 2.6.30):
> 
> 	CONFIG_TREE_RCU:
> 
> 	   text	   data	    bss	    dec	    filename
> 	    663      32      20     715     kernel/rcupdate.o
> 	   3278     528      44    3850     kernel/rcutree.o
> 				   4565 Total (vs 4045 for v4)
> 
> 	CONFIG_TREE_PREEMPT_RCU:
> 
> 	   text	   data	    bss	    dec	    filename
> 	    743      32      20     795     kernel/rcupdate.o
> 	   4548     752      60    5360     kernel/rcutree.o
> 	   			   6155 Total (N/A for v4)
> 
> 	CONFIG_TINY_RCU:
> 
> 	   text	   data	    bss	    dec	    filename
> 	     96       4       0     100     kernel/rcupdate.o
> 	    720      28       0     748     kernel/rcutiny.o
> 	    			    848 Total (vs 1140 for v6)
> 
> The above is for x86.  Your mileage may vary on other platforms.
> Further compression is possible, but is being procrastinated.
> 
> Changes from v6 (http://lkml.org/lkml/2009/9/23/293).
> 
> o	Forward ported to put it into the 2.6.33 stream.
> 
> o	Added lockdep support.
> 
> o	Make lightweight rcu_barrier.
> 
> Changes from v5 (http://lkml.org/lkml/2009/6/23/12).
> 
> o	Ported to latest pre-2.6.32 merge window kernel.
> 
> 	- Renamed rcu_qsctr_inc() to rcu_sched_qs().
> 	- Renamed rcu_bh_qsctr_inc() to rcu_bh_qs().
> 	- Provided trivial rcu_cpu_notify().
> 	- Provided trivial exit_rcu().
> 	- Provided trivial rcu_needs_cpu().
> 	- Fixed up the rcu_*_enter/exit() functions in linux/hardirq.h.
> 
> o	Removed the dependence on EMBEDDED, with a view to making
> 	TINY_RCU default for !SMP at some time in the future.
> 
> o	Added (trivial) support for expedited grace periods.
> 
> Changes from v4 (http://lkml.org/lkml/2009/5/2/91) include:
> 
> o	Squeeze the size down a bit further by removing the
> 	->completed field from struct rcu_ctrlblk.
> 
> o	This permits synchronize_rcu() to become the empty function.
> 	Previous concerns about rcutorture were unfounded, as
> 	rcutorture correctly handles a constant value from
> 	rcu_batches_completed() and rcu_batches_completed_bh().
> 
> Changes from v3 (http://lkml.org/lkml/2009/3/29/221) include:
> 
> o	Changed rcu_batches_completed(), rcu_batches_completed_bh()
> 	rcu_enter_nohz(), rcu_exit_nohz(), rcu_nmi_enter(), and
> 	rcu_nmi_exit(), to be static inlines, as suggested by David
> 	Howells.  Doing this saves about 100 bytes from rcutiny.o.
> 	(The numbers between v3 and this v4 of the patch are not directly
> 	comparable, since they are against different versions of Linux.)
> 
> Changes from v2 (http://lkml.org/lkml/2009/2/3/333) include:
> 
> o	Fix whitespace issues.
> 
> o	Change short-circuit "||" operator to instead be "+" in order to
> 	fix performance bug noted by "kraai" on LWN.
> 
> 		(http://lwn.net/Articles/324348/)
> 
> Changes from v1 (http://lkml.org/lkml/2009/1/13/440) include:
> 
> o	This version depends on EMBEDDED as well as !SMP, as suggested
> 	by Ingo.
> 
> o	Updated rcu_needs_cpu() to unconditionally return zero,
> 	permitting the CPU to enter dynticks-idle mode at any time.
> 	This works because callbacks can be invoked upon entry to
> 	dynticks-idle mode.
> 
> o	Paul is now OK with this being included, based on a poll at the
> 	Kernel Miniconf at linux.conf.au, where about ten people said
> 	that they cared about saving 900 bytes on single-CPU systems.
> 
> o	Applies to both mainline and tip/core/rcu.
> 
> Signed-off-by: David Howells <dhowells@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> ---
>  include/linux/hardirq.h  |   24 ++++
>  include/linux/rcupdate.h |    6 +
>  include/linux/rcutiny.h  |  103 +++++++++++++++++
>  init/Kconfig             |    9 ++
>  kernel/Makefile          |    1 +
>  kernel/rcupdate.c        |    4 +
>  kernel/rcutiny.c         |  281 ++++++++++++++++++++++++++++++++++++++++++++++
>  7 files changed, 428 insertions(+), 0 deletions(-)
>  create mode 100644 include/linux/rcutiny.h
>  create mode 100644 kernel/rcutiny.c
> 
> diff --git a/include/linux/hardirq.h b/include/linux/hardirq.h
> index 6d527ee..d5b3876 100644
> --- a/include/linux/hardirq.h
> +++ b/include/linux/hardirq.h
> @@ -139,10 +139,34 @@ static inline void account_system_vtime(struct task_struct *tsk)
>  #endif
>  
>  #if defined(CONFIG_NO_HZ)
> +#if defined(CONFIG_TINY_RCU)
> +extern void rcu_enter_nohz(void);
> +extern void rcu_exit_nohz(void);
> +
> +static inline void rcu_irq_enter(void)
> +{
> +	rcu_exit_nohz();
> +}
> +
> +static inline void rcu_irq_exit(void)
> +{
> +	rcu_enter_nohz();
> +}
> +
> +static inline void rcu_nmi_enter(void)
> +{
> +}
> +
> +static inline void rcu_nmi_exit(void)
> +{
> +}
> +
> +#else
>  extern void rcu_irq_enter(void);
>  extern void rcu_irq_exit(void);
>  extern void rcu_nmi_enter(void);
>  extern void rcu_nmi_exit(void);
> +#endif
>  #else
>  # define rcu_irq_enter() do { } while (0)
>  # define rcu_irq_exit() do { } while (0)
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 3ebd0b7..6dd71fa 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -68,11 +68,17 @@ extern int sched_expedited_torture_stats(char *page);
>  /* Internal to kernel */
>  extern void rcu_init(void);
>  extern void rcu_scheduler_starting(void);
> +#ifndef CONFIG_TINY_RCU
>  extern int rcu_needs_cpu(int cpu);
> +#else
> +static inline int rcu_needs_cpu(int cpu) { return 0; }
> +#endif
>  extern int rcu_scheduler_active;
>  
>  #if defined(CONFIG_TREE_RCU) || defined(CONFIG_TREE_PREEMPT_RCU)
>  #include <linux/rcutree.h>
> +#elif CONFIG_TINY_RCU
> +#include <linux/rcutiny.h>
>  #else
>  #error "Unknown RCU implementation specified to kernel configuration"
>  #endif
> diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
> new file mode 100644
> index 0000000..08f17ab
> --- /dev/null
> +++ b/include/linux/rcutiny.h
> @@ -0,0 +1,103 @@
> +/*
> + * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition.
> + *
> + * 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, 2008
> + *
> + * Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> + *
> + * For detailed explanation of Read-Copy Update mechanism see -
> + * 		Documentation/RCU
> + */
> +
> +#ifndef __LINUX_TINY_H
> +#define __LINUX_TINY_H
> +
> +#include <linux/cache.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. */
> +};
> +
> +extern struct rcu_ctrlblk rcu_ctrlblk;
> +extern struct rcu_ctrlblk rcu_bh_ctrlblk;
> +
> +void rcu_sched_qs(int cpu);
> +void rcu_bh_qs(int cpu);
> +
> +#define __rcu_read_lock()	preempt_disable()
> +#define __rcu_read_unlock()	preempt_enable()
> +#define __rcu_read_lock_bh()	local_bh_disable()
> +#define __rcu_read_unlock_bh()	local_bh_enable()
> +#define call_rcu_sched		call_rcu
> +
> +#define rcu_init_sched()	do { } while (0)
> +extern void rcu_check_callbacks(int cpu, int user);
> +extern void __rcu_init(void);
> +/* extern void rcu_restart_cpu(int cpu); */
> +
> +/*
> + * Return the number of grace periods.
> + */
> +static inline long rcu_batches_completed(void)
> +{
> +	return 0;
> +}
> +
> +/*
> + * Return the number of bottom-half grace periods.
> + */
> +static inline long rcu_batches_completed_bh(void)
> +{
> +	return 0;
> +}
> +
> +extern int rcu_expedited_torture_stats(char *page);
> +
> +static inline int rcu_pending(int cpu)
> +{
> +	return 1;
> +}
> +
> +struct notifier_block;
> +extern int rcu_cpu_notify(struct notifier_block *self,
> +			  unsigned long action, void *hcpu);
> +
> +#ifdef CONFIG_NO_HZ
> +
> +extern void rcu_enter_nohz(void);
> +extern void rcu_exit_nohz(void);
> +
> +#else /* #ifdef CONFIG_NO_HZ */
> +
> +static inline void rcu_enter_nohz(void)
> +{
> +}
> +
> +static inline void rcu_exit_nohz(void)
> +{
> +}
> +
> +#endif /* #else #ifdef CONFIG_NO_HZ */
> +
> +static inline void exit_rcu(void)
> +{
> +}
> +
> +#endif /* __LINUX_RCUTINY_H */
> diff --git a/init/Kconfig b/init/Kconfig
> index 0121c0e..4fecb53 100644
> --- a/init/Kconfig
> +++ b/init/Kconfig
> @@ -334,6 +334,15 @@ config TREE_PREEMPT_RCU
>  	  is also required.  It also scales down nicely to
>  	  smaller systems.
>  
> +config TINY_RCU
> +	bool "UP-only small-memory-footprint RCU"
> +	depends on !SMP
> +	help
> +	  This option selects the RCU implementation that is
> +	  designed for UP systems from which real-time response
> +	  is not required.  This option greatly reduces the
> +	  memory footprint of RCU.
> +
>  endchoice
>  
>  config RCU_TRACE
> diff --git a/kernel/Makefile b/kernel/Makefile
> index 7c9b0a5..0098bcf 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -83,6 +83,7 @@ obj-$(CONFIG_RCU_TORTURE_TEST) += rcutorture.o
>  obj-$(CONFIG_TREE_RCU) += rcutree.o
>  obj-$(CONFIG_TREE_PREEMPT_RCU) += rcutree.o
>  obj-$(CONFIG_TREE_RCU_TRACE) += rcutree_trace.o
> +obj-$(CONFIG_TINY_RCU) += rcutiny.o
>  obj-$(CONFIG_RELAY) += relay.o
>  obj-$(CONFIG_SYSCTL) += utsname_sysctl.o
>  obj-$(CONFIG_TASK_DELAY_ACCT) += delayacct.o
> diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c
> index 4001833..7625f20 100644
> --- a/kernel/rcupdate.c
> +++ b/kernel/rcupdate.c
> @@ -67,6 +67,8 @@ void wakeme_after_rcu(struct rcu_head  *head)
>  	complete(&rcu->completion);
>  }
>  
> +#ifndef CONFIG_TINY_RCU
> +
>  #ifdef CONFIG_TREE_PREEMPT_RCU
>  
>  /**
> @@ -157,6 +159,8 @@ void synchronize_rcu_bh(void)
>  }
>  EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
>  
> +#endif /* #ifndef CONFIG_TINY_RCU */
> +
>  static int __cpuinit rcu_barrier_cpu_hotplug(struct notifier_block *self,
>  		unsigned long action, void *hcpu)
>  {
> diff --git a/kernel/rcutiny.c b/kernel/rcutiny.c
> new file mode 100644
> index 0000000..89124b0
> --- /dev/null
> +++ b/kernel/rcutiny.c
> @@ -0,0 +1,281 @@
> +/*
> + * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition.
> + *
> + * 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, 2008
> + *
> + * Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> + *
> + * 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/rcupdate.h>
> +#include <linux/interrupt.h>
> +#include <linux/sched.h>
> +#include <linux/module.h>
> +#include <linux/completion.h>
> +#include <linux/moduleparam.h>
> +#include <linux/notifier.h>
> +#include <linux/cpu.h>
> +#include <linux/mutex.h>
> +#include <linux/time.h>
> +
> +/* Definition for rcupdate control block. */
> +struct rcu_ctrlblk rcu_ctrlblk = {
> +	.rcucblist = NULL,
> +	.donetail = &rcu_ctrlblk.rcucblist,
> +	.curtail = &rcu_ctrlblk.rcucblist,
> +};
> +EXPORT_SYMBOL_GPL(rcu_ctrlblk);
> +struct rcu_ctrlblk rcu_bh_ctrlblk = {
> +	.rcucblist = NULL,
> +	.donetail = &rcu_bh_ctrlblk.rcucblist,
> +	.curtail = &rcu_bh_ctrlblk.rcucblist,
> +};
> +EXPORT_SYMBOL_GPL(rcu_bh_ctrlblk);
> +
> +#ifdef CONFIG_NO_HZ
> +
> +static long rcu_dynticks_nesting = 1;
> +
> +/*
> + * Enter dynticks-idle mode, which is an extended quiescent state
> + * if we have fully entered that mode (i.e., if the new value of
> + * dynticks_nesting is zero).
> + */
> +void rcu_enter_nohz(void)
> +{
> +	if (--rcu_dynticks_nesting == 0)
> +		rcu_sched_qs(0); /* implies rcu_bh_qsctr_inc(0) */
> +}
> +
> +/*
> + * Exit dynticks-idle mode, so that we are no longer in an extended
> + * quiescent state.
> + */
> +void rcu_exit_nohz(void)
> +{
> +	rcu_dynticks_nesting++;
> +}
> +
> +#endif /* #ifdef CONFIG_NO_HZ */
> +
> +/*
> + * Helper function for rcu_qsctr_inc() and rcu_bh_qsctr_inc().
> + */
> +static int rcu_qsctr_help(struct rcu_ctrlblk *rcp)
> +{
> +	if (rcp->rcucblist != NULL &&
> +	    rcp->donetail != rcp->curtail) {
> +		rcp->donetail = rcp->curtail;
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +/*
> + * Record an rcu quiescent state.  And an rcu_bh quiescent state while we
> + * are at it, given that any rcu quiescent state is also an rcu_bh
> + * quiescent state.  Use "+" instead of "||" to defeat short circuiting.
> + */
> +void rcu_sched_qs(int cpu)
> +{
> +	if (rcu_qsctr_help(&rcu_ctrlblk) + rcu_qsctr_help(&rcu_bh_ctrlblk))
> +		raise_softirq(RCU_SOFTIRQ);


local_irq_disable()(better) or local_bh_disable() is needed here.

see here:
schedule() {
	...
	preempt_disable();
	....
	rcu_sched_qs(cpu); /* nothing to proctect accessing rcp->donetail */
	.....
}

> +}
> +
> +/*
> + * Record an rcu_bh quiescent state.
> + */
> +void rcu_bh_qs(int cpu)
> +{
> +	if (rcu_qsctr_help(&rcu_bh_ctrlblk))
> +		raise_softirq(RCU_SOFTIRQ);


It doesn't need local_irq_disable() nor local_bh_disable().
It's only called at __do_softirq(), but maybe a comment is needed.

> +}
> +
> +/*
> + * Check to see if the scheduling-clock interrupt came from an extended
> + * quiescent state, and, if so, tell RCU about it.
> + */
> +void rcu_check_callbacks(int cpu, int user)
> +{
> +	if (!rcu_needs_cpu(0))
> +		return;	/* RCU doesn't need anything to be done. */

rcu_needs_cpu(0) always returns 0 ......
The next statements will not be executed .....

> +	if (user ||
> +	    (idle_cpu(cpu) &&
> +	     !in_softirq() &&
> +	     hardirq_count() <= (1 << HARDIRQ_SHIFT)))
> +		rcu_sched_qs(cpu);
> +	else if (!in_softirq())
> +		rcu_bh_qs(cpu);
> +}
> +
> +/*
> + * Helper function for rcu_process_callbacks() that operates on the
> + * specified rcu_ctrlkblk structure.
> + */
> +static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp)
> +{
> +	unsigned long flags;
> +	struct rcu_head *next, *list;
> +
> +	/* If no RCU callbacks ready to invoke, just return. */
> +	if (&rcp->rcucblist == rcp->donetail)
> +		return;
> +
> +	/* Move the ready-to-invoke callbacks to a local list. */
> +	local_irq_save(flags);
> +	list = rcp->rcucblist;
> +	rcp->rcucblist = *rcp->donetail;
> +	*rcp->donetail = NULL;
> +	if (rcp->curtail == rcp->donetail)
> +		rcp->curtail = &rcp->rcucblist;
> +	rcp->donetail = &rcp->rcucblist;
> +	local_irq_restore(flags);
> +
> +	/* Invoke the callbacks on the local list. */
> +	while (list) {
> +		next = list->next;
> +		prefetch(next);
> +		list->func(list);
> +		list = next;
> +	}
> +}
> +
> +/*
> + * Invoke any callbacks whose grace period has completed.
> + */
> +static void rcu_process_callbacks(struct softirq_action *unused)
> +{
> +	__rcu_process_callbacks(&rcu_ctrlblk);
> +	__rcu_process_callbacks(&rcu_bh_ctrlblk);
> +}
> +
> +/*
> + * Null function to handle CPU being onlined.  Longer term, we want to
> + * make TINY_RCU avoid using rcupdate.c, but later...
> + */
> +int rcu_cpu_notify(struct notifier_block *self,
> +		   unsigned long action, void *hcpu)
> +{
> +	return NOTIFY_OK;
> +}
> +
> +/*
> + * Wait for a grace period to elapse.  But it is illegal to invoke
> + * synchronize_sched() from within an RCU read-side critical section.
> + * Therefore, any legal call to synchronize_sched() is a quiescent
> + * state, and so on a UP system, synchronize_sched() need do nothing.
> + * Ditto for synchronize_rcu_bh().
> + *
> + * Cool, huh?  (Due to Josh Triplett.)
> + *
> + * But we want to make this a static inline later.
> + */
> +void synchronize_sched(void)
> +{

I stubbornly recommend adding a cond_resched()/might_sleep() here.

It reduces latency. (for !CONFIG_PREEMPT)
It prevents someone calls it on nonsleepable context.

> +}
> +EXPORT_SYMBOL_GPL(synchronize_sched);
> +
> +void synchronize_rcu_bh(void)
> +{

Ditto.

> +}
> +EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
> +
> +/*
> + * Helper function for call_rcu() and call_rcu_bh().
> + */
> +static void __call_rcu(struct rcu_head *head,
> +		       void (*func)(struct rcu_head *rcu),
> +		       struct rcu_ctrlblk *rcp)
> +{
> +	unsigned long flags;
> +
> +	head->func = func;
> +	head->next = NULL;
> +	local_irq_save(flags);
> +	*rcp->curtail = head;
> +	rcp->curtail = &head->next;
> +	local_irq_restore(flags);
> +}
> +
> +/*
> + * Post an RCU callback to be invoked after the end of an RCU grace
> + * period.  But since we have but one CPU, that would be after any
> + * quiescent state.
> + */
> +void call_rcu(struct rcu_head *head,
> +	      void (*func)(struct rcu_head *rcu))
> +{
> +	__call_rcu(head, func, &rcu_ctrlblk);
> +}
> +EXPORT_SYMBOL_GPL(call_rcu);
> +
> +/*
> + * Post an RCU bottom-half callback to be invoked after any subsequent
> + * quiescent state.
> + */
> +void call_rcu_bh(struct rcu_head *head,
> +		 void (*func)(struct rcu_head *rcu))
> +{
> +	__call_rcu(head, func, &rcu_bh_ctrlblk);
> +}
> +EXPORT_SYMBOL_GPL(call_rcu_bh);
> +
> +void rcu_barrier(void)
> +{
> +	struct rcu_synchronize rcu;
> +
> +	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(rcu_barrier);
> +
> +void rcu_barrier_bh(void)
> +{
> +	struct rcu_synchronize rcu;
> +
> +	init_completion(&rcu.completion);
> +	/* Will wake me after RCU finished. */
> +	call_rcu_bh(&rcu.head, wakeme_after_rcu);
> +	/* Wait for it. */
> +	wait_for_completion(&rcu.completion);
> +}
> +EXPORT_SYMBOL_GPL(rcu_barrier_bh);
> +
> +void rcu_barrier_sched(void)
> +{
> +	struct rcu_synchronize rcu;
> +
> +	init_completion(&rcu.completion);
> +	/* Will wake me after RCU finished. */
> +	call_rcu_sched(&rcu.head, wakeme_after_rcu);
> +	/* Wait for it. */
> +	wait_for_completion(&rcu.completion);


alternative implementation(nonsleep implementation)

{
	cond_resched();

	rcp = &rcu_ctrlblk;
	local_irq_save(flags);
	if (rcp->rcucblist != NULL) {
		rcp->donetail = rcp->curtail;
		local_irq_restore(flags);

		local_bh_disable();
		__rcu_process_callbacks(rcp);
		local_bh_enable();
	} else
		local_irq_restore(flags);
}

Ditto for other rcu_barrier*()

> +}
> +EXPORT_SYMBOL_GPL(rcu_barrier_sched);
> +
> +void __rcu_init(void)
> +{
> +	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
> +}

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