From 159720704d9d652b64390c11fb971e15b0a78d23 Mon Sep 17 00:00:00 2001 From: Sunil Khatri Date: Tue, 8 Sep 2026 14:47:29 +0530 Subject: [PATCH] drm/drm_exec: fix up contended obj when num_objects is 0 MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit drm_exec_prepare_array() silently returns success without calling drm_exec_lock_contended() when num_objects is zero. This breaks the invariant upheld by drm_exec_lock_obj(), where every entry point into the locking sequence must first attempt to lock any previously contended object before proceeding. Drivers that chain multiple drm_exec_prepare_array() calls per drm_exec_until_all_locked() iteration (e.g. amdgpu's userq signal/wait ioctls, which prepare separate read and write BO arrays) can pass an empty array for one of the two calls. If contention is hit while preparing the non-empty array, exec->contended is set and the loop retries; on retry, the empty-array call preceding it is a no-op that never clears exec->contended, so drm_exec_retry_on_contention() immediately jumps back to the top of the loop without ever reaching the call that would resolve the contention. This spins forever. Fix it by having drm_exec_prepare_array() call drm_exec_lock_contended() directly when num_objects is zero, so a pending contended object dont loop infinitely. Fixes: 09593216bff1 ("drm: execution context for GEM buffers v7") CC: stable@vger.kernel.org # v6.6+ Signed-off-by: Sunil Khatri Link: https://lore.kernel.org/r/20260908091729.2749399-1-sunil.khatri@amd.com Reviewed-by: Christian König Signed-off-by: Christian König --- drivers/gpu/drm/drm_exec.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) diff --git a/drivers/gpu/drm/drm_exec.c b/drivers/gpu/drm/drm_exec.c index 41034a5996ff..2453ec41360f 100644 --- a/drivers/gpu/drm/drm_exec.c +++ b/drivers/gpu/drm/drm_exec.c @@ -322,6 +322,19 @@ int drm_exec_prepare_array(struct drm_exec *exec, { int ret; + /* + * Make sure to lock a contended object even when no objects are + * given, otherwise drm_exec_retry_on_contention() would loop + * forever on patterns like: + * + * ret = drm_exec_prepare_array(exec, objs, num_objects, ...); + * drm_exec_retry_on_contention(exec); + * + * with num_objects == 0. + */ + if (!num_objects) + return drm_exec_lock_contended(exec); + for (unsigned int i = 0; i < num_objects; ++i) { ret = drm_exec_prepare_obj(exec, objects[i], num_fences); if (unlikely(ret))