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);
}
