On Tue, Aug 24, 2021 at 04:31:21PM -0700, Daniele Ceraolo Spurio wrote:
>
>
> On 8/18/2021 11:16 PM, Matthew Brost wrote:
> > If the context is reset as a result of the request cancelation the
> > context reset G2H is received after schedule disable done G2H which is
> > likely the wrong order. The schedule disable done G2H release the
> > waiting request cancelation code which resubmits the context. This races
> > with the context reset G2H which also wants to resubmit the context but
> > in this case it really should be a NOP as request cancelation code owns
> > the resubmit. Use some clever tricks of checking the context state to
> > seal this race until if / when the GuC firmware is fixed.
>
> Did you raise this with the GuC team? If it's a GuC issue we definitely want
> a fix there ASAP so we can drop any i915-side WAs.
>
Yep, def an issue with the GuC firmware behavior. Will get fixed, just
not sure when.
> >
> > v2:
> > (Checkpatch)
> > - Fix typos
> >
> > Fixes: 62eaf0ae217d ("drm/i915/guc: Support request cancellation")
> > Signed-off-by: Matthew Brost <matthew.brost@xxxxxxxxx>
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > ---
> > .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 43 ++++++++++++++++---
> > 1 file changed, 37 insertions(+), 6 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 e4a099f8f820..8f7a11e65ef5 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -832,17 +832,35 @@ __unwind_incomplete_requests(struct intel_context *ce)
> > static void __guc_reset_context(struct intel_context *ce, bool stalled)
> > {
> > struct i915_request *rq;
> > + unsigned long flags;
> > u32 head;
> > + bool skip = false;
> > intel_context_get(ce);
> > /*
> > - * GuC will implicitly mark the context as non-schedulable
> > - * when it sends the reset notification. Make sure our state
> > - * reflects this change. The context will be marked enabled
> > - * on resubmission.
> > + * GuC will implicitly mark the context as non-schedulable when it sends
> > + * the reset notification. Make sure our state reflects this change. The
> > + * context will be marked enabled on resubmission.
> > + *
> > + * XXX: If the context is reset as a result of the request cancellation
> > + * this G2H is received after the schedule disable complete G2H which is
> > + * likely wrong as this creates a race between the request cancellation
> > + * code re-submitting the context and this G2H handler. This likely
> > + * should be fixed in the GuC but until if / when that gets fixed we
> > + * need to workaround this. Convert this function to a NOP if a pending
> > + * enable is in flight as this indicates that a request cancellation has
> > + * occurred.
> > */
>
> IMO this comment sounds like we're not clear on expected behavior. Either
> the ordering is wrong, in which case we have a GuC bug and this is a
> temporary WA, or the ordering is allowed and we need to cope with it. The
> way the comment is written sounds like we're not sure.
>
Comments written prior to confirmation that GuC behavior was wrong, will
reword.
> Code changes look ok.
>
Ty. I'll think we have to carry this until we upgrade the GuC firmware
with a the proper behavior - until then without this workaround
canceling non-preemptable requests is 100% broken, hence why I added a
selftest. Will add a FIXME / XXX comment so we can remove this in the
future.
Matt
> Daniele
>
> > - clr_context_enabled(ce);
> > + spin_lock_irqsave(&ce->guc_state.lock, flags);
> > + if (likely(!context_pending_enable(ce))) {
> > + clr_context_enabled(ce);
> > + } else {
> > + skip = true;
> > + }
> > + spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> > + if (unlikely(skip))
> > + goto out_put;
> > rq = intel_context_find_active_request(ce);
> > if (!rq) {
> > @@ -861,6 +879,7 @@ static void __guc_reset_context(struct intel_context *ce, bool stalled)
> > out_replay:
> > guc_reset_state(ce, head, stalled);
> > __unwind_incomplete_requests(ce);
> > +out_put:
> > intel_context_put(ce);
> > }
> > @@ -1605,6 +1624,13 @@ static void guc_context_cancel_request(struct intel_context *ce,
> > guc_reset_state(ce, intel_ring_wrap(ce->ring, rq->head),
> > true);
> > }
> > +
> > + /*
> > + * XXX: Racey if context is reset, see comment in
> > + * __guc_reset_context().
> > + */
> > + flush_work(&ce_to_guc(ce)->ct.requests.worker);
> > +
> > guc_context_unblock(ce);
> > }
> > }
> > @@ -2719,7 +2745,12 @@ static void guc_handle_context_reset(struct intel_guc *guc,
> > {
> > trace_intel_context_reset(ce);
> > - if (likely(!intel_context_is_banned(ce))) {
> > + /*
> > + * XXX: Racey if request cancellation has occurred, see comment in
> > + * __guc_reset_context().
> > + */
> > + if (likely(!intel_context_is_banned(ce) &&
> > + !context_blocked(ce))) {
> > capture_error_state(guc, ce);
> > guc_context_replay(ce);
> > }
>
