[BUG] "sched: Remove rq->lock from the first half of ttwu()" locks up on ARM

[yong.zhang0@gmail.com (Yong Zhang)]

On Thu, May 26, 2011 at 5:15 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> On Wed, 2011-05-25 at 19:08 +0200, Peter Zijlstra wrote:
>> Ooh, shiny, whilst typing this I got an NMI-watchdog error reporting me
>> that CPU1 got stuck in try_to_wake_up(), so it looks like I can indeed
>> reproduce some funnies.
>>
>> /me goes dig in.
>
> Does the below make your ARM box happy again?
>
> It restores the old ttwu behaviour for this case and seems to not mess
> up my x86 with __ARCH_WANT_INTERRUPTS_ON_CTXSW.
>
> Figuring out why the existing condition failed

Seems  'current' will change before/after switch_to since it's derived from
sp register.
So that means if interrupt come before we switch sp, 'p == current' will
catch it, but if interrupt comes after we switch sp, we will lose a wake up.

Thanks,
Yong

> and writing a proper
> changelog requires a mind that is slightly less deprived of sleep and I
> shall attempt that tomorrow -- provided this does indeed work for you.
>
> ---
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 2d12893..6976eac 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2573,7 +2573,19 @@ static void ttwu_queue_remote(struct task_struct *p, int cpu)
> ? ? ? ?if (!next)
> ? ? ? ? ? ? ? ?smp_send_reschedule(cpu);
> ?}
> -#endif
> +
> +#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> +static void ttwu_activate_remote(struct task_struct *p, int wake_flags)
> +{
> + ? ? ? struct rq *rq = __task_rq_lock(p);
> +
> + ? ? ? ttwu_activate(rq, p, ENQUEUE_WAKEUP | ENQUEUE_WAKING);
> + ? ? ? ttwu_do_wakeup(rq, p, wake_flags);
> +
> + ? ? ? __task_rq_unlock(rq);
> +}
> +#endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
> +#endif /* CONFIG_SMP */
>
> ?static void ttwu_queue(struct task_struct *p, int cpu)
> ?{
> @@ -2630,18 +2642,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
> ? ? ? ? */
> ? ? ? ?while (p->on_cpu) {
> ?#ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> - ? ? ? ? ? ? ? /*
> - ? ? ? ? ? ? ? ?* If called from interrupt context we could have landed in the
> - ? ? ? ? ? ? ? ?* middle of schedule(), in this case we should take care not
> - ? ? ? ? ? ? ? ?* to spin on ->on_cpu if p is current, since that would
> - ? ? ? ? ? ? ? ?* deadlock.
> - ? ? ? ? ? ? ? ?*/
> - ? ? ? ? ? ? ? if (p == current) {
> - ? ? ? ? ? ? ? ? ? ? ? ttwu_queue(p, cpu);
> - ? ? ? ? ? ? ? ? ? ? ? goto stat;
> - ? ? ? ? ? ? ? }
> -#endif
> + ? ? ? ? ? ? ? ttwu_activate_remote(p, wake_flags);
> + ? ? ? ? ? ? ? goto stat;
> +#else
> ? ? ? ? ? ? ? ?cpu_relax();
> +#endif
> ? ? ? ?}
> ? ? ? ?/*
> ? ? ? ? * Pairs with the smp_wmb() in finish_lock_switch().
>
>
> --
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