[Intel-gfx] [bug report] drm/i915/guc: Implement multi-lrc submission

[Dan Carpenter]

On Thu, Sep 22, 2022 at 01:01:48PM -0700, John Harrison wrote:
> On 9/22/2022 07:26, Dan Carpenter wrote:
> > Hello Matthew Brost,
> > 
> > The patch 6b540bf6f143: "drm/i915/guc: Implement multi-lrc
> > submission" from Oct 14, 2021, leads to the following Smatch static
> > checker warning:
> > 
> > 	drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c:752 __guc_add_request()
> > 	warn: refcount leak 'ce->ref.refcount.refs.counter': lines='752'
> > 
> > drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> >      672 static int __guc_add_request(struct intel_guc *guc, struct i915_request *rq)
> >      673 {
> >      674         int err = 0;
> >      675         struct intel_context *ce = request_to_scheduling_context(rq);
> >      676         u32 action[3];
> >      677         int len = 0;
> >      678         u32 g2h_len_dw = 0;
> >      679         bool enabled;
> >      680
> >      681         lockdep_assert_held(&rq->engine->sched_engine->lock);
> >      682
> >      683         /*
> >      684          * Corner case where requests were sitting in the priority list or a
> >      685          * request resubmitted after the context was banned.
> >      686          */
> >      687         if (unlikely(intel_context_is_banned(ce))) {
> >      688                 i915_request_put(i915_request_mark_eio(rq));
> >      689                 intel_engine_signal_breadcrumbs(ce->engine);
> >      690                 return 0;
> >      691         }
> >      692
> >      693         GEM_BUG_ON(!atomic_read(&ce->guc_id.ref));
> >      694         GEM_BUG_ON(context_guc_id_invalid(ce));
> >      695
> >      696         if (context_policy_required(ce)) {
> >      697                 err = guc_context_policy_init_v70(ce, false);
> >      698                 if (err)
> >      699                         return err;
> >      700         }
> >      701
> >      702         spin_lock(&ce->guc_state.lock);
> >      703
> >      704         /*
> >      705          * The request / context will be run on the hardware when scheduling
> >      706          * gets enabled in the unblock. For multi-lrc we still submit the
> >      707          * context to move the LRC tails.
> >      708          */
> >      709         if (unlikely(context_blocked(ce) && !intel_context_is_parent(ce)))
> >      710                 goto out;
> >      711
> >      712         enabled = context_enabled(ce) || context_blocked(ce);
> >      713
> >      714         if (!enabled) {
> >      715                 action[len++] = INTEL_GUC_ACTION_SCHED_CONTEXT_MODE_SET;
> >      716                 action[len++] = ce->guc_id.id;
> >      717                 action[len++] = GUC_CONTEXT_ENABLE;
> >      718                 set_context_pending_enable(ce);
> >      719                 intel_context_get(ce);
> >      720                 g2h_len_dw = G2H_LEN_DW_SCHED_CONTEXT_MODE_SET;
> >      721         } else {
> >      722                 action[len++] = INTEL_GUC_ACTION_SCHED_CONTEXT;
> >      723                 action[len++] = ce->guc_id.id;
> >      724         }
> >      725
> >      726         err = intel_guc_send_nb(guc, action, len, g2h_len_dw);
> >      727         if (!enabled && !err) {
> >      728                 trace_intel_context_sched_enable(ce);
> >      729                 atomic_inc(&guc->outstanding_submission_g2h);
> >      730                 set_context_enabled(ce);
> >      731
> >      732                 /*
> >      733                  * Without multi-lrc KMD does the submission step (moving the
> >      734                  * lrc tail) so enabling scheduling is sufficient to submit the
> >      735                  * context. This isn't the case in multi-lrc submission as the
> >      736                  * GuC needs to move the tails, hence the need for another H2G
> >      737                  * to submit a multi-lrc context after enabling scheduling.
> >      738                  */
> >      739                 if (intel_context_is_parent(ce)) {
> >      740                         action[0] = INTEL_GUC_ACTION_SCHED_CONTEXT;
> >      741                         err = intel_guc_send_nb(guc, action, len - 1, 0);
> > 
> > Should this have a something like:
> > 
> > 				if (err)
> > 					intel_context_put(ce);
> No.
> 
> The ce has been marked as enabled above because the actual enable call
> succeeded.? This is a secondary call to 'poke' the context. If this fails,
> the context might not get scheduled in a timely manner but the tracking
> state is still correct. The context is now in use by GuC and therefore must
> be reference locked. And the 'context_enabled' flag is set on the i915 side
> to note that the reference count is held.
> 
> If you want to unwind at this point, you would need to send a
> CONTEXT_MODE_SET(DISABLE) H2G message (amongst other things) and only once
> that call has completed, can you call intel_context_put(ce).
> 
> I don't get why the analyser would be claiming a leak here. I'm not sure if
> it is just that the analyser is confused or if there is some other potential
> route to a leak. Is it possible to get more details as to how it thinks the
> leak can occur?

Thanks, this helps me a lot!

This is a Smatch static analysis thing I'm working on.  It simple enough
to silence the false positive.  I add this line which says that after
set_context_enabled() then ignore the reference counting.

	add_function_param_key_hook("set_context_enabled", &match_ignore,
			0, "$->ref.refcount.refs.counter", NULL);

The heuristic that the check is using is that if some error paths drop
the refcount then all error paths should do it.  So adding the if
statement I suggested would silence the warning (but introduce a bug in
the kernel).

regards,
dan carpenter