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

diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index ef0cc08f5dfb740112c1d3456e2c08d58b868cbc..38cc7389a6c172c4f22ac89ec3694220a2083dc1 100644 (file)
--- a/drivers/dma-buf/sw_sync.c
+++ b/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);