Re: [PATCH tip/core/rcu 10/26] rcu: Exit RCU extended QS on kernelpreemption after irq/exception

[Josh Triplett]

On Thu, Aug 30, 2012 at 02:05:27PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@xxxxxxxxx>
> 
> 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@xxxxxxxxx>
> Cc: Alessio Igor Bogani <abogani@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Avi Kivity <avi@xxxxxxxxxx>
> Cc: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Cc: Christoph Lameter <cl@xxxxxxxxx>
> Cc: Geoff Levand <geoff@xxxxxxxxxxxxx>
> Cc: Gilad Ben Yossef <gilad@xxxxxxxxxxxxx>
> Cc: Hakan Akkan <hakanakkan@xxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Kevin Hilman <khilman@xxxxxx>
> Cc: Max Krasnyansky <maxk@xxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Stephen Hemminger <shemminger@xxxxxxxxxx>
> Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>

Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>

>  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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