dma-fence: fix dma_fence_get_rcu_safe v2

When dma_fence_get_rcu() fails to acquire a reference it doesn't necessary
mean that there is no fence at all.

It usually mean that the fence was replaced by a new one and in this situation
we certainly want to have the new one as result and *NOT* NULL.

v2: Keep extra check after dma_fence_get_rcu().

Signed-off-by: Christian König <christian.koenig@amd.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: Sumit Semwal <sumit.semwal@linaro.org>
Cc: linux-media@vger.kernel.org
Cc: dri-devel@lists.freedesktop.org
Cc: linaro-mm-sig@lists.linaro.org
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1505469187-3565-1-git-send-email-deathsimple@vodafone.de

include/linux/dma-fence.h

@@ -248,9 +248,12 @@ dma_fence_get_rcu_safe(struct dma_fence * __rcu *fencep)
 		struct dma_fence *fence;
 
 		fence = rcu_dereference(*fencep);
-		if (!fence || !dma_fence_get_rcu(fence))
+		if (!fence)
 			return NULL;
 
+		if (!dma_fence_get_rcu(fence))
+			continue;
+
 		/* The atomic_inc_not_zero() inside dma_fence_get_rcu()
 		 * provides a full memory barrier upon success (such as now).
 		 * This is paired with the write barrier from assigning