On 5/29/26 23:52, Philipp Stanner wrote:
> drm_sched contains a lockless queue (spsc_queue) that seems to be
> useless and potentially unsound.
>
> Add a TODO list entry for replacing spsc_queue with a locked list.
>
> Signed-off-by: Philipp Stanner <[email protected]>
Reviewed-by: Christian König <[email protected]>
> ---
> Documentation/gpu/todo.rst | 41 ++++++++++++++++++++++++++++++++++++++
> 1 file changed, 41 insertions(+)
>
> diff --git a/Documentation/gpu/todo.rst b/Documentation/gpu/todo.rst
> index cdddf8db35f5..87e082b0eb48 100644
> --- a/Documentation/gpu/todo.rst
> +++ b/Documentation/gpu/todo.rst
> @@ -948,6 +948,47 @@ Contact: Philipp Stanner <[email protected]>
>
> Level: Intermediate
>
> +Replace the lockless queue with a locked list
> +---------------------------------------------
> +
> +drm_sched is the only user in the entire kernel of a special lockless queue,
> the
> +spsc_queue. This queue utilizes:
> +
> +- preempt_disable()
> +- atomic instructions
> +- memory barriers
> +- ACCESS_ONCE()
> +
> +whereas a conventional spinlock utilizes:
> +
> +- preempt_disable()
> +- 1 atomic instruction for taking / releasing the lock
> +- memory barriers
> +
> +Moreover, drm_sched_entity_push_job(), the only user of spsc_queue_push(),
> has
> +to take a lock in some situations anyways and calls to it are often
> serialized
> +with a driver lock.
> +
> +It is, thus, highly questionable whether the lockless queue grants any
> advantage
> +at all. Considering that its internals are not well documented and its
> correctness
> +is not formally proven, it seems desirable to replace the queue with a mere
> list
> +or hlist that is protected by a spinlock.
> +
> +Tasks:
> +
> +- Replace the spsc_queue in drm/sched (and those who might access the
> scheduler's
> + internal queue) with a spinlock + (h)list.
> +- Ideally, check with some micro benchmarks and real world tests (preferably
> + with amdgpu) for relevant performance regressions.
> +- Remove the spsc_queue from the kernel altogether.
> +
> +Contact:
> +
> +- Philipp Stanner <[email protected]>
> +- Christian König <[email protected]>
> +
> +Level: Beginner
> +
> Outside DRM
> ===========
>
