[PATCH 6.6 287/299] drm/syncobj: handle NULL fence in syncobj_eventfd_entry_func

Greg Kroah-Hartman
2024-02-27T14:00:13Z

From: Erik Kurzinger <ekurzinger@nvidia.com>

[ Upstream commit 2aa6f5b0fd052e363bb9d4b547189f0bf6b3d6d3 ]

During syncobj_eventfd_entry_func, dma_fence_chain_find_seqno may set
the fence to NULL if the given seqno is signaled and a later seqno has
already been submitted. In that case, the eventfd should be signaled
immediately which currently does not happen.

This is a similar issue to the one addressed by commit b19926d4f3a6
("drm/syncobj: Deal with signalled fences in drm_syncobj_find_fence.").

As a fix, if the return value of dma_fence_chain_find_seqno indicates
success but it sets the fence to NULL, we will assign a stub fence to
ensure the following code still signals the eventfd.

v1 -> v2: assign a stub fence instead of signaling the eventfd

Signed-off-by: Erik Kurzinger <ekurzinger@nvidia.com>
Fixes: c7a472297169 ("drm/syncobj: add IOCTL to register an eventfd")
Signed-off-by: Simon Ser <contact@emersion.fr>
Link: https://patchwork.freedesktop.org/patch/msgid/20240221184527.37667-1-ekurzinger@nvidia.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
drivers/gpu/drm/drm_syncobj.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index b3433265be6ab..5860428da8de8 100644
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1380,10 +1380,21 @@ syncobj_eventfd_entry_func(struct drm_syncobj *syncobj,

/* This happens inside the syncobj lock */
fence = dma_fence_get(rcu_dereference_protected(syncobj->fence, 1));
+ if (!fence)
+ return;
+
ret = dma_fence_chain_find_seqno(&fence, entry->point);
- if (ret != 0 || !fence) {
+ if (ret != 0) {
+ /* The given seqno has not been submitted yet. */
dma_fence_put(fence);
return;
+ } else if (!fence) {
+ /* If dma_fence_chain_find_seqno returns 0 but sets the fence
+ * to NULL, it implies that the given seqno is signaled and a
+ * later seqno has already been submitted. Assign a stub fence
+ * so that the eventfd still gets signaled below.
+ */
+ fence = dma_fence_get_stub();
}

list_del_init(&entry->node);
--
2.43.0