linux-kernel - Re: [PATCH tip/core/rcu 10/26] rcu: Exit RCU extended QS on kernel preemption after irq/exception

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Message-ID: <20120831235157.GG11771@jtriplet-mobl1>
Date:	Fri, 31 Aug 2012 16:51:57 -0700
From:	Josh Triplett <josh@...htriplett.org>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...ymtl.ca, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, sbw@....edu, patches@...aro.org,
	Alessio Igor Bogani <abogani@...nel.org>,
	Avi Kivity <avi@...hat.com>,
	Chris Metcalf <cmetcalf@...era.com>,
	Christoph Lameter <cl@...ux.com>,
	Geoff Levand <geoff@...radead.org>,
	Gilad Ben Yossef <gilad@...yossef.com>,
	Hakan Akkan <hakanakkan@...il.com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Kevin Hilman <khilman@...com>,
	Max Krasnyansky <maxk@...lcomm.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	Sven-Thorsten Dietrich <thebigcorporation@...il.com>
Subject: Re: [PATCH tip/core/rcu 10/26] rcu: Exit RCU extended QS on kernel
 preemption after irq/exception

On Thu, Aug 30, 2012 at 02:05:27PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@...il.com>
> 
> When an exception or an irq exits, and we are going to resume into
> interrupted kernel code, the low level architecture code calls
> preempt_schedule_irq() if there is a need to reschedule.
> 
> If the interrupt/exception occured between a call to rcu_user_enter()
> (from syscall exit, exception exit, do_notify_resume exit, ...) and
> a real resume to userspace (iret,...), preempt_schedule_irq() can be
> called whereas RCU thinks we are in userspace. But preempt_schedule_irq()
> is going to run kernel code and may be some RCU read side critical
> section. We must exit the userspace extended quiescent state before
> we call it.
> 
> To solve this, just call rcu_user_exit() in the beginning of
> preempt_schedule_irq().
> 
> Signed-off-by: Frederic Weisbecker <fweisbec@...il.com>
> Cc: Alessio Igor Bogani <abogani@...nel.org>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Avi Kivity <avi@...hat.com>
> Cc: Chris Metcalf <cmetcalf@...era.com>
> Cc: Christoph Lameter <cl@...ux.com>
> Cc: Geoff Levand <geoff@...radead.org>
> Cc: Gilad Ben Yossef <gilad@...yossef.com>
> Cc: Hakan Akkan <hakanakkan@...il.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Josh Triplett <josh@...htriplett.org>
> Cc: Kevin Hilman <khilman@...com>
> Cc: Max Krasnyansky <maxk@...lcomm.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Stephen Hemminger <shemminger@...tta.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@...il.com>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@...htriplett.org>

>  kernel/sched/core.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 07c6d9a..0bd599b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3564,6 +3564,7 @@ asmlinkage void __sched preempt_schedule_irq(void)
>  	/* Catch callers which need to be fixed */
>  	BUG_ON(ti->preempt_count || !irqs_disabled());
>  
> +	rcu_user_exit();
>  	do {
>  		add_preempt_count(PREEMPT_ACTIVE);
>  		local_irq_enable();
> -- 
> 1.7.8
> 
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