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

Josh Triplett
2012-08-31T23:52:09Z

On Thu, Aug 30, 2012 at 02:05:27PM -0700, Paul E. McKenney wrote:
> From: Frederic Weisbecker <fweisbec@gmail.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@gmail.com>
> Cc: Alessio Igor Bogani <abogani@kernel.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Chris Metcalf <cmetcalf@tilera.com>
> Cc: Christoph Lameter <cl@linux.com>
> Cc: Geoff Levand <geoff@infradead.org>
> Cc: Gilad Ben Yossef <gilad@benyossef.com>
> Cc: Hakan Akkan <hakanakkan@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Kevin Hilman <khilman@ti.com>
> Cc: Max Krasnyansky <maxk@qualcomm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Stephen Hemminger <shemminger@vyatta.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Sven-Thorsten Dietrich <thebigcorporation@gmail.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

Reviewed-by: Josh Triplett <josh@joshtriplett.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
>