drm/syncobj: handle NULL fence in syncobj_eventfd_entry_func

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

diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c
index 56b0677acfa3da9f31f9e7a6a3c97764c5248f13..a6c19de462928ed70da033a60b10f08061bd1dc8 100644 (file)
--- a/drivers/gpu/drm/drm_syncobj.c
+++ b/drivers/gpu/drm/drm_syncobj.c
@@ -1418,10 +1418,21 @@ 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);