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

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