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Subject[PATCH 4.9 096/109] dma-buf/sw_sync: clean up list before signaling the fence
4.9-stable review patch.  If anyone has any objections, please let me know.


From: Gustavo Padovan <>

commit 3792b7c1a70815fe4e954221c096f9278638fd21 upstream.

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 <>
Signed-off-by: Gustavo Padovan <>
Reviewed-by: Chris Wilson <>
[s/dma_fence/fence/g - gregkh]
Cc: Jisheng Zhang <>
Signed-off-by: Greg Kroah-Hartman <>

drivers/dma-buf/sw_sync.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)

--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -213,11 +213,21 @@ static void sync_timeline_signal(struct
obj->value += inc;

list_for_each_entry_safe(pt, next, &obj->pt_list, link) {
- if (!fence_is_signaled_locked(&pt->base))
+ if (!timeline_fence_signaled(&pt->base))

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().
+ */
+ fence_signal_locked(&pt->base);


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