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

[Jani Nikula]

On Wed, 07 Jan 2015, Chris Wilson <chris at chris-wilson.co.uk> wrote:
> On Wed, Jan 07, 2015 at 02:12:14PM +0200, Jani Nikula wrote:
>> On Mon, 05 Jan 2015, Daniel Vetter <daniel at 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 at chris-wilson.co.uk>
>> >> Cc: stable at vger.kernel.org
>> >
>> > Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>
>> 
>> Pushed to drm-intel-fixes, thanks for the patch and review.
>
> For the record, I plan to post a revert of this to -next if the core fix
> lands upstream (looks good so far).

Does it look like that could be cc:stable?

BR,
Jani.

> -Chris
>
> -- 
> Chris Wilson, Intel Open Source Technology Centre

-- 
Jani Nikula, Intel Open Source Technology Center