Re: [PATCH] Documentation: drm: Add entry for removing spsc_queue to TODO list

[Philipp Stanner]

On Mon, 2026-06-01 at 10:30 +0200, Christian König wrote:
> 
> 
> 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 <phasta@xxxxxxxxxx>
> 
> Reviewed-by: Christian König <christian.koenig@xxxxxxx>

Pushed to drm-misc-next

Thx
P.

> 
> > ---
> >  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 <phasta@xxxxxxxxxx>
> >  
> >  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 <phasta@xxxxxxxxxx>
> > +- Christian König <christian.koenig@xxxxxxx>
> > +
> > +Level: Beginner
> > +
> >  Outside DRM
> >  ===========
> >