android: unconditionally remove callbacks in sync_fence_free()

BugLink: https://bugs.launchpad.net/bugs/1826212

[ Upstream commit 699f6855 ]

Using fence->status to determine whether or not there are callbacks
remaining on the sync_fence is racy since fence->status may have been
decremented to 0 on another CPU before fence_check_cb_func() has
completed.  By unconditionally calling fence_remove_callback() for each
fence in the sync_fence, we guarantee that each callback has either
completed (since fence_remove_callback() grabs the fence lock) or been
removed.
Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Signed-off-by: Dmitry Torokhov <dtor@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>

drivers/staging/android/sync.c

@@ -519,12 +519,10 @@ static const struct fence_ops android_fence_ops = {
 static void sync_fence_free(struct kref *kref)
 {
 	struct sync_fence *fence = container_of(kref, struct sync_fence, kref);
-	int i, status = atomic_read(&fence->status);
+	int i;
 
 	for (i = 0; i < fence->num_fences; ++i) {
-		if (status)
-			fence_remove_callback(fence->cbs[i].sync_pt,
-					      &fence->cbs[i].cb);
+		fence_remove_callback(fence->cbs[i].sync_pt, &fence->cbs[i].cb);
 		fence_put(fence->cbs[i].sync_pt);
 	}