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SubjectRe: Possible bug with mutex adaptative spinning
From
Date

>
> I've queued the below patch

Thanks. Should it make -stable as well ?

Cheers,
Ben.

> ---
> Subject: mutex: Don't spin when the owner CPU is offline or other weird cases
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Date: Fri Apr 16 23:20:00 CEST 2010
>
> Due to recent load-balancer changes that delay the task migration to
> the next wakeup, the adaptive mutex spinning ends up in a live lock
> when the owner's CPU gets offlined because the cpu_online() check
> lives before the owner running check.
>
> This patch changes mutex_spin_on_owner() to return 0 (don't spin) in
> any case where we aren't sure about the owner struct validity or CPU
> number, and if the said CPU is offline. There is no point going back &
> re-evaluate spinning in corner cases like that, let's just go to
> sleep.
>
> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
> LKML-Reference: <1271212509.13059.135.camel@pasglop>
> ---
> kernel/sched.c | 8 ++++----
> 1 file changed, 4 insertions(+), 4 deletions(-)
>
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -3647,7 +3647,7 @@ int mutex_spin_on_owner(struct mutex *lo
> * the mutex owner just released it and exited.
> */
> if (probe_kernel_address(&owner->cpu, cpu))
> - goto out;
> + return 0;
> #else
> cpu = owner->cpu;
> #endif
> @@ -3657,14 +3657,14 @@ int mutex_spin_on_owner(struct mutex *lo
> * the cpu field may no longer be valid.
> */
> if (cpu >= nr_cpumask_bits)
> - goto out;
> + return 0;
>
> /*
> * We need to validate that we can do a
> * get_cpu() and that we have the percpu area.
> */
> if (!cpu_online(cpu))
> - goto out;
> + return 0;
>
> rq = cpu_rq(cpu);
>
> @@ -3683,7 +3683,7 @@ int mutex_spin_on_owner(struct mutex *lo
>
> cpu_relax();
> }
> -out:
> +
> return 1;
> }
> #endif
>




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