staging: android: sync: Signal pt before sync_timeline object gets destroyed

There is a race condition

Assume we have *one* sync_fence object, with *one* sync_pt
which belongs to *one* sync_timeline, given this condition,
sync_timeline->kref will have two counts, one for sync_timeline
(implicit) and another for sync_pt.

Assume following is the situation on CPU

Theead-1 : (Thread which calls sync_timeline_destroy())
  -> (some function calls)
   -> sync_timeline_destory()
    -> sync_timeline_signal() (CPU is inside this
function after putting reference to sync_timeline)

At this time Thread-2 comes and does following

Thread-2 : (fclose on fence fd)
> sync_fence_release() -> because of fclose() on fence object
 -> sync_fence_free()
  -> sync_pt_free()
   -> kref_put(&pt->parent->kref, sync_timeline_free);
    -> sync_timeline_free() (CPU is inside this because
this time kref will be zero after _put)

Thread-2 will free sync_timeline object before Thread-1
has finished its work inside sync_timeline_signal.

With this change we signals all sync_pt before putting
reference to sync_timeline object.

Cc: Colin Cross <ccross@android.com>
Cc: Android Kernel Team <kernel-team@android.com>
Signed-off-by: Prakash Kamliya <pkamliya@codeaurora.org>
[jstultz: minor commit subject tweak]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/android/sync.c

@@ -92,14 +92,14 @@ static void sync_timeline_free(struct kref *kref)
 void sync_timeline_destroy(struct sync_timeline *obj)
 {
 	obj->destroyed = true;
+	smp_wmb();
 
 	/*
-	 * If this is not the last reference, signal any children
-	 * that their parent is going away.
+	 * signal any children that their parent is going away.
 	 */
+	sync_timeline_signal(obj);
 
-	if (!kref_put(&obj->kref, sync_timeline_free))
-		sync_timeline_signal(obj);
+	kref_put(&obj->kref, sync_timeline_free);
 }
 EXPORT_SYMBOL(sync_timeline_destroy);