[PATCH v3] dma-buf/sync_file: Increment refcount of fence when all are signaled.

[Rafael Antognolli]

When we merge several fences, if all of them are signaled already, we
still keep one of them. So instead of using add_fence(), which will not
increase the refcount of signaled fences, we should explicitly call
fence_get() for the fence we are keeping.

This patch fixes a kernel panic that can be triggered by creating a fence that
is expired (or increasing the timeline until it expires), then creating a
merged fence out of it, and deleting the merged fence. This will make the
original expired fence's refcount go to zero.

Testcase: igt/sw_sync/sync_expired_merge
Signed-off-by: Rafael Antognolli <rafael.antognolli at intel.com>
Reviewed-by: Chris Wilson <chris at chris-wilson.co.uk>
Reviewed-by: Gustavo Padovan <gustavo.padovan at collabora.co.uk>
---
 drivers/dma-buf/sync_file.c | 6 ++----
 1 file changed, 2 insertions(+), 4 deletions(-)

diff --git a/drivers/dma-buf/sync_file.c b/drivers/dma-buf/sync_file.c
index abb5fda..0fe7ec2 100644
--- a/drivers/dma-buf/sync_file.c
+++ b/drivers/dma-buf/sync_file.c
@@ -253,10 +253,8 @@ static struct sync_file *sync_file_merge(const char *name, struct sync_file *a,
 	for (; i_b < b_num_fences; i_b++)
 		add_fence(fences, &i, b_fences[i_b]);
 
-	if (i == 0) {
-		add_fence(fences, &i, a_fences[0]);
-		i++;
-	}
+	if (i == 0)
+		fences[i++] = fence_get(a_fences[0]);
 
 	if (num_fences > i) {
 		nfences = krealloc(fences, i * sizeof(*fences),
-- 
2.7.4