drm/nouveau: Avoid potential race between nouveau_fence_update() and context takedown.

Signed-off-by: Francisco Jerez <currojerez@riseup.net>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

drivers/gpu/drm/nouveau/nouveau_fence.c

@@ -77,6 +77,8 @@ nouveau_fence_update(struct nouveau_channel *chan)
 
 	spin_lock(&chan->fence.lock);
 
+	/* Fetch the last sequence if the channel is still up and running */
+	if (likely(!list_empty(&chan->fence.pending))) {
 		if (USE_REFCNT(dev))
 			sequence = nvchan_rd32(chan, 0x48);
 		else
@@ -85,6 +87,7 @@ nouveau_fence_update(struct nouveau_channel *chan)
 		if (chan->fence.sequence_ack == sequence)
 			goto out;
 		chan->fence.sequence_ack = sequence;
+	}
 
 	list_for_each_entry_safe(fence, tmp, &chan->fence.pending, entry) {
 		sequence = fence->sequence;