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drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

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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>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
This commit is contained in:
Chris Wilson 2015-01-02 09:47:10 +00:00 committed by Jani Nikula
parent 48bf5b2d00
commit 226e5ae9e5
1 changed files with 1 additions and 1 deletions
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2
drivers/gpu/drm/i915/i915_gem.c
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@ -5155,7 +5155,7 @@ 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 */