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

Peter Zijlstra
2012-09-06T16:54:21Z

On Thu, 2012-08-30 at 14:05 -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().

> ---
> 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?