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authorChris Wilson <chris@chris-wilson.co.uk>2015-01-02 09:47:10 +0000
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>2015-01-29 17:40:55 -0800
commitb78dc65fd7e4fb2cd2ca6ef6866eb2989fb8ee6c (patch)
tree5cca015403e22e0192d8cb7800d4b4dd74ff77f6
parent0d9928f365c2bbbab82a3f56c0c612343cd260d1 (diff)
downloadlwn-b78dc65fd7e4fb2cd2ca6ef6866eb2989fb8ee6c.tar.gz
lwn-b78dc65fd7e4fb2cd2ca6ef6866eb2989fb8ee6c.zip
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 <chris@chris-wilson.co.uk> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch> Signed-off-by: Jani Nikula <jani.nikula@intel.com> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
-rw-r--r--drivers/gpu/drm/i915/i915_gem.c2
1 files changed, 1 insertions, 1 deletions
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 */