Re: [PATCH 2/2] drm/sched: document the RCU dependency

[Philipp Stanner <phasta@mailbox.org>]

On Wed, 2026-09-09 at 15:14 +0200, Christian König wrote:
> Tvrkos patches added RCU protection to the returned strings from
> get_timeline_name()/get_driver_name() callbacks of the dma_fence
> backends.
> 
> This fixed use after free problems for a couple of drivers, but we never
> documented the consequences for the drm_sched_fence.
> 
> Add a few words on the function documentation to note that we need an
> RCU grace period between signaling the last scheduler fence and
> scheduler teardown.
> 
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/gpu/drm/scheduler/sched_main.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/gpu/drm/scheduler/sched_main.c b/drivers/gpu/drm/scheduler/sched_main.c
> index 6cb6f9546493..22103cb07782 100644
> --- a/drivers/gpu/drm/scheduler/sched_main.c
> +++ b/drivers/gpu/drm/scheduler/sched_main.c
> @@ -1203,6 +1203,10 @@ static void drm_sched_cancel_remaining_jobs(struct drm_gpu_scheduler *sched)
>   * is implemented, all jobs will be canceled through it and afterwards cleaned
>   * up through &struct drm_sched_backend_ops.free_job. If cancel_job is not
>   * implemented, memory could leak.
> + *
> + * The scheduler fences timeline name is returned protected by the signaled
> + * status and RCU, so an RCU grace period is necessary between signaling the
> + * last scheduler fence and tearing down the scheduler who originated it.
>   */

The API user does not signal a "scheduler fence", typically. Are you
referring to the hardware-fence?

The user must signal all hardware-fences, which will cause all
finished-fences (which are at the root of the addressed problem) to be
signaled.

So what you probably want to say is "The user must wait one RCU grace
period between signaling the last hardware-fence and calling this
function because …".


P.