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

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Date:	Thu, 06 Sep 2012 18:52:44 +0200
From:	Peter Zijlstra <peterz@...radead.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, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, 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, 2012-08-30 at 14:05 -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().

> ---
>  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();

But in this case you want to restart the tick as well, are we going to
add a nohz_user_exit() like call as well, or do we have something that
does both?
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