dma-buf/sw_sync: clean up list before signaling the fence

If userspace already dropped its own reference by closing the sw_sync
fence fd we might end up in a deadlock where
dma_fence_is_signaled_locked() will trigger the release of the fence and
thus try to hold the lock to remove the fence from the list.

dma_fence_is_signaled_locked() tries to release/free the fence and hold
the lock in the process.

We fix that by changing the order operation and clean up the list and
rb-tree first.

v2: Drop fence get/put dance and manipulate the list first (Chris Wilson)

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NGustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170729152217.8362-2-gustavo@padovan.org

drivers/dma-buf/sw_sync.c

@@ -213,11 +213,21 @@ static void sync_timeline_signal(struct sync_timeline *obj, unsigned int inc)
 	obj->value += inc;
 
 	list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
-		if (!dma_fence_is_signaled_locked(&pt->base))
+		if (!timeline_fence_signaled(&pt->base))
 			break;
 
 		list_del_init(&pt->link);
 		rb_erase(&pt->node, &obj->pt_tree);
+
+		/*
+		 * A signal callback may release the last reference to this
+		 * fence, causing it to be freed. That operation has to be
+		 * last to avoid a use after free inside this loop, and must
+		 * be after we remove the fence from the timeline in order to
+		 * prevent deadlocking on timeline->lock inside
+		 * timeline_fence_release().
+		 */
+		dma_fence_signal_locked(&pt->base);
 	}
 
 	spin_unlock_irq(&obj->lock);