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drm/drm_exec: fix up contended obj when num_objects is 0

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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: 09593216bf ("drm: execution context for GEM buffers v7")
CC: stable@vger.kernel.org # v6.6+
Signed-off-by: Sunil Khatri <sunil.khatri@amd.com>
Link: https://lore.kernel.org/r/20260908091729.2749399-1-sunil.khatri@amd.com
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
This commit is contained in:
Sunil Khatri 2026-09-08 14:47:29 +05:30 committed by Christian König
parent f97802dd98
commit 159720704d
1 changed files with 13 additions and 0 deletions
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13
drivers/gpu/drm/drm_exec.c
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@ -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))