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

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On 6/5/2024 5:17 PM, Andi Shyti wrote:
The ce->guc_state.lock was made to protect guc_prio, which
indicates the GuC priority level.

But at the begnning of the function we perform some sanity check
of guc_prio outside its protected section. Move them within the
locked region.

Use this occasion to expand the if statement to make it clearer.

Fixes: ee242ca704d3 ("drm/i915/guc: Implement GuC priority management")
Signed-off-by: Andi Shyti <andi.shyti@xxxxxxxxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v5.15+
---
  drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c | 15 +++++++++++----
  1 file changed, 11 insertions(+), 4 deletions(-)

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.

- 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.

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.

Daniele

  		if (rq->guc_prio != GUC_PRIO_INIT)
  			sub_context_inflight_prio(ce, rq->guc_prio);
@@ -4281,6 +4286,8 @@ static void guc_bump_inflight_request_prio(struct i915_request *rq,
  		add_context_inflight_prio(ce, rq->guc_prio);
  		update_context_prio(ce);
  	}
+
+exit:
  	spin_unlock(&ce->guc_state.lock);
  }




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