Possible bug with mutex adaptative spinning

On Wed, 2010-04-14 at 12:35 +1000, Benjamin Herrenschmidt wrote:

> [PATCH] mutex: Don't spin when the owner CPU is offline or other weird cases
> 
> The current code might spin forever if the CPU owning the mutex has been
> offlined, and the last CPU in the system is trying to acquire it, since
> mutex_spin_on_owner() will always return 1, telling the caller to spin
> until the mutex has been released.
> 
> 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>

Right, patch looks good, comments are a tad misleading.


I've queued the below patch

---
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(-)

> 
> 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
>