Re: [PATCH] drm/i915/gt/uc: Evaluate GuC priority within locks

[Andi Shyti <andi.shyti@linux.intel.com>]

Hi Daniele,

thanks for checking this patch.

> > diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > index 0eaa1064242c..1181043bc5e9 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -4267,13 +4267,18 @@ static void guc_bump_inflight_request_prio(struct i915_request *rq,
> >   	u8 new_guc_prio = map_i915_prio_to_guc_prio(prio);
> >   	/* Short circuit function */
> > -	if (prio < I915_PRIORITY_NORMAL ||
> > -	    rq->guc_prio == GUC_PRIO_FINI ||
> > -	    (rq->guc_prio != GUC_PRIO_INIT &&
> > -	     !new_guc_prio_higher(rq->guc_prio, new_guc_prio)))
> > +	if (prio < I915_PRIORITY_NORMAL)
> >   		return;
> 
> My understanding was that those checks are purposely done outside of the
> lock to avoid taking it when not needed and that the early exit is not racy.
> In particular:
> 
> - GUC_PRIO_FINI is the end state for the priority, so if we're there that's
> not changing anymore and therefore the lock is not required.

yeah... then I thought that the lock should either remove it
completely or have everything inside the lock.

> - the priority only goes up with the bumping, so if new_guc_prio_higher() is
> false that's not going to be changed by a different thread running at the
> same time and increasing the priority even more.
> 
> I think there is still a possible race is if new_guc_prio_higher() is true
> when we check it outside the lock but then changes before we execute the
> protected chunk inside, so a fix would still be required for that.

This is the reason why I made the patch :-)

> All this said, I don't really have anything against moving the whole thing
> inside the lock since this isn't on a critical path, just wanted to point
> out that it's not all strictly required.
> 
> One nit on the code below.
> 
> >   	spin_lock(&ce->guc_state.lock);
> > +
> > +	if (rq->guc_prio == GUC_PRIO_FINI)
> > +		goto exit;
> > +
> > +	if (rq->guc_prio != GUC_PRIO_INIT &&
> > +	    !new_guc_prio_higher(rq->guc_prio, new_guc_prio))
> > +		goto exit;
> > +
> >   	if (rq->guc_prio != GUC_PRIO_FINI) {
> 
> You're now checking for rq->guc_prio == GUC_PRIO_FINI inside the lock, so no
> need to check it again here as it can't have changed.

True, will resend.

Thanks, Daniele!

Andi