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

[Jani Nikula <jani.nikula@linux.intel.com>]

On Mon, 05 Jan 2015, Daniel Vetter <daniel@ffwll.ch> wrote:
> 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>

Pushed to drm-intel-fixes, thanks for the patch and review.

BR,
Jani.

>
>> ---
>>  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
>> 
>> _______________________________________________
>> Intel-gfx mailing list
>> Intel-gfx@lists.freedesktop.org
>> http://lists.freedesktop.org/mailman/listinfo/intel-gfx
>
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> +41 (0) 79 365 57 48 - http://blog.ffwll.ch

-- 
Jani Nikula, Intel Open Source Technology Center