From: Chris Wilson Date: Fri, 2 Jan 2015 09:47:10 +0000 (+0000) Subject: drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES X-Git-Tag: firefly_0821_release~3679^2~920 X-Git-Url: http://demsky.eecs.uci.edu/git/?a=commitdiff_plain;h=b78dc65fd7e4fb2cd2ca6ef6866eb2989fb8ee6c;p=firefly-linux-kernel-4.4.55.git drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES commit 226e5ae9e5f9108beb0bde4ac69f68fe6210fed9 upstream. 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 Reviewed-by: Daniel Vetter Signed-off-by: Jani Nikula Signed-off-by: Greg Kroah-Hartman --- diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c index 0a30088178b0..0b71a0aaf4fc 100644 --- a/drivers/gpu/drm/i915/i915_gem.c +++ b/drivers/gpu/drm/i915/i915_gem.c @@ -4449,7 +4449,7 @@ 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 */