[PATCH] dma-buf/sw_sync: hold a fence reference when check if it signaled

[Gustavo Padovan]

From: Gustavo Padovan <gustavo.padovan at collabora.com>

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 a
thus try to hold the lock to remove the fence from the list.

We need to grab a reference to the fence before calling into this chain if
we want to avoid this issue.

Cc: Chris Wilson <chris at chris-wilson.co.uk>
Signed-off-by: Gustavo Padovan <gustavo.padovan at collabora.com>
---
 drivers/dma-buf/sw_sync.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/sw_sync.c b/drivers/dma-buf/sw_sync.c
index af1bc84..8291434 100644
--- a/drivers/dma-buf/sw_sync.c
+++ b/drivers/dma-buf/sw_sync.c
@@ -144,11 +144,16 @@ 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))
+		dma_fence_get(&pt->base);
+		if (!dma_fence_is_signaled_locked(&pt->base)) {
+			dma_fence_put(&pt->base);
 			break;
+		}
 
 		list_del_init(&pt->link);
 		rb_erase(&pt->node, &obj->pt_tree);
+
+		dma_fence_put(&pt->base);
 	}
 
 	spin_unlock_irq(&obj->lock);
-- 
2.9.4