Re: [PATCH] drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

[Daniel Vetter <daniel@ffwll.ch>]

On Fri, Jan 02, 2015 at 09:47:10AM +0000, Chris Wilson wrote:
> If CONFIG_DEBUG_MUTEXES is set, the mutex->owner field is only cleared
> if the mutex debugging is enabled which introduces a race in our
> mutex_is_locked_by() - i.e. we may inspect the old owner value before it
> is acquired by the new task.
> 
> This is the root cause of this error:
> 
> diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
> index 5cf6731..3ef3736 100644
> --- a/kernel/locking/mutex-debug.c
> +++ b/kernel/locking/mutex-debug.c
> @@ -80,13 +80,13 @@ void debug_mutex_unlock(struct mutex *lock)
>                         DEBUG_LOCKS_WARN_ON(lock->owner != current);
> 
>                 DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
> -               mutex_clear_owner(lock);
>         }
> 
>         /*
>          * __mutex_slowpath_needs_to_unlock() is explicitly 0 for debug
>          * mutexes so that we can do it here after we've verified state.
>          */
> +       mutex_clear_owner(lock);
>         atomic_set(&lock->count, 1);
>  }
> 
> Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87955
> Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> Cc: stable@vger.kernel.org

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

> ---
>  drivers/gpu/drm/i915/i915_gem.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
> index ad55b06a3cb1..4c14db9587a6 100644
> --- a/drivers/gpu/drm/i915/i915_gem.c
> +++ b/drivers/gpu/drm/i915/i915_gem.c
> @@ -5024,7 +5024,7 @@ static bool mutex_is_locked_by(struct mutex *mutex, struct task_struct *task)
>  	if (!mutex_is_locked(mutex))
>  		return false;
>  
> -#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_MUTEXES)
> +#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES)
>  	return mutex->owner == task;
>  #else
>  	/* Since UP may be pre-empted, we cannot assume that we own the lock */
> -- 
> 2.1.4
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
+41 (0) 79 365 57 48 - http://blog.ffwll.ch
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