Re: [Intel-gfx] [PATCH 21/27] drm/i915/guc: Proper xarray usage for contexts_lookup

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Aug 25, 2021 at 05:44:11PM -0700, Daniele Ceraolo Spurio wrote:
> 
> 
> On 8/18/2021 11:16 PM, Matthew Brost wrote:
> > Lock the xarray and take ref to the context if needed.
> > 
> > v2:
> >   (Checkpatch)
> >    - Add new line after declaration
> >   (Daniel Vetter)
> >    - Correct put / get accounting in xa_for_loops
> > 
> > Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> > ---
> >   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 103 +++++++++++++++---
> >   1 file changed, 88 insertions(+), 15 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 509b298e7cf3..5f77f25322ca 100644
> > --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> > @@ -606,8 +606,18 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
> >   	unsigned long index, flags;
> >   	bool pending_disable, pending_enable, deregister, destroyed, banned;
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> >   	xa_for_each(&guc->context_lookup, index, ce) {
> > -		spin_lock_irqsave(&ce->guc_state.lock, flags);
> > +		/*
> > +		 * Corner case where the ref count on the object is zero but and
> > +		 * deregister G2H was lost. In this case we don't touch the ref
> > +		 * count and finish the destroy of the context.
> > +		 */
> > +		bool do_put = kref_get_unless_zero(&ce->ref);
> > +
> > +		xa_unlock(&guc->context_lookup);
> > +
> > +		spin_lock(&ce->guc_state.lock);
> >   		/*
> >   		 * Once we are at this point submission_disabled() is guaranteed
> > @@ -623,7 +633,9 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
> >   		banned = context_banned(ce);
> >   		init_sched_state(ce);
> > -		spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> > +		spin_unlock(&ce->guc_state.lock);
> > +
> > +		GEM_BUG_ON(!do_put && !destroyed);
> >   		if (pending_enable || destroyed || deregister) {
> >   			decr_outstanding_submission_g2h(guc);
> > @@ -646,13 +658,19 @@ static void scrub_guc_desc_for_outstanding_g2h(struct intel_guc *guc)
> >   			}
> >   			intel_context_sched_disable_unpin(ce);
> >   			decr_outstanding_submission_g2h(guc);
> > -			spin_lock_irqsave(&ce->guc_state.lock, flags);
> > +
> > +			spin_lock(&ce->guc_state.lock);
> >   			guc_blocked_fence_complete(ce);
> > -			spin_unlock_irqrestore(&ce->guc_state.lock, flags);
> > +			spin_unlock(&ce->guc_state.lock);
> >   			intel_context_put(ce);
> >   		}
> > +
> > +		if (do_put)
> > +			intel_context_put(ce);
> 
> is it safe to do the put outside the xa_lock, in case the refcount goes to
> zero with this? I know it is unlikely because the refcount was > 0 if do_put
> is true, but it might've gone down between us checking earlier and now.
> 

It is safe as xa_for_each indicates it is safe to destroy / delete
objects from the array while traversing it. 

> > +		xa_lock(&guc->context_lookup);
> >   	}
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> >   }
> >   static inline bool
> > @@ -873,16 +891,29 @@ void intel_guc_submission_reset(struct intel_guc *guc, bool stalled)
> >   {
> >   	struct intel_context *ce;
> >   	unsigned long index;
> > +	unsigned long flags;
> >   	if (unlikely(!guc_submission_initialized(guc))) {
> >   		/* Reset called during driver load? GuC not yet initialised! */
> >   		return;
> >   	}
> > -	xa_for_each(&guc->context_lookup, index, ce)
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> > +	xa_for_each(&guc->context_lookup, index, ce) {
> > +		if (!kref_get_unless_zero(&ce->ref))
> > +			continue;
> > +
> > +		xa_unlock(&guc->context_lookup);
> > +
> >   		if (intel_context_is_pinned(ce))
> >   			__guc_reset_context(ce, stalled);
> > +		intel_context_put(ce);
> > +
> > +		xa_lock(&guc->context_lookup);
> > +	}
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> > +
> >   	/* GuC is blown away, drop all references to contexts */
> >   	xa_destroy(&guc->context_lookup);
> >   }
> > @@ -957,11 +988,24 @@ void intel_guc_submission_cancel_requests(struct intel_guc *guc)
> >   {
> >   	struct intel_context *ce;
> >   	unsigned long index;
> > +	unsigned long flags;
> > +
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> > +	xa_for_each(&guc->context_lookup, index, ce) {
> > +		if (!kref_get_unless_zero(&ce->ref))
> > +			continue;
> > +
> > +		xa_unlock(&guc->context_lookup);
> > -	xa_for_each(&guc->context_lookup, index, ce)
> >   		if (intel_context_is_pinned(ce))
> >   			guc_cancel_context_requests(ce);
> > +		intel_context_put(ce);
> > +
> > +		xa_lock(&guc->context_lookup);
> > +	}
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> > +
> >   	guc_cancel_sched_engine_requests(guc->sched_engine);
> >   	/* GuC is blown away, drop all references to contexts */
> > @@ -2850,21 +2894,28 @@ void intel_guc_find_hung_context(struct intel_engine_cs *engine)
> >   	struct intel_context *ce;
> >   	struct i915_request *rq;
> >   	unsigned long index;
> > +	unsigned long flags;
> >   	/* Reset called during driver load? GuC not yet initialised! */
> >   	if (unlikely(!guc_submission_initialized(guc)))
> >   		return;
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> >   	xa_for_each(&guc->context_lookup, index, ce) {
> > -		if (!intel_context_is_pinned(ce))
> > +		if (!kref_get_unless_zero(&ce->ref))
> >   			continue;
> > +		xa_unlock(&guc->context_lookup);
> > +
> > +		if (!intel_context_is_pinned(ce))
> > +			goto next;
> > +
> >   		if (intel_engine_is_virtual(ce->engine)) {
> >   			if (!(ce->engine->mask & engine->mask))
> > -				continue;
> > +				goto next;
> >   		} else {
> >   			if (ce->engine != engine)
> > -				continue;
> > +				goto next;
> >   		}
> >   		list_for_each_entry(rq, &ce->guc_active.requests, sched.link) {
> > @@ -2874,9 +2925,17 @@ void intel_guc_find_hung_context(struct intel_engine_cs *engine)
> >   			intel_engine_set_hung_context(engine, ce);
> >   			/* Can only cope with one hang at a time... */
> > -			return;
> > +			intel_context_put(ce);
> > +			xa_lock(&guc->context_lookup);
> > +			goto done;
> >   		}
> > +next:
> > +		intel_context_put(ce);
> > +		xa_lock(&guc->context_lookup);
> > +
> 
> nit: extra newline
> 

Checkpatch got that one too. Already fixed.

Matt

> Daniele
> 
> >   	}
> > +done:
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> >   }
> >   void intel_guc_dump_active_requests(struct intel_engine_cs *engine,
> > @@ -2892,23 +2951,34 @@ void intel_guc_dump_active_requests(struct intel_engine_cs *engine,
> >   	if (unlikely(!guc_submission_initialized(guc)))
> >   		return;
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> >   	xa_for_each(&guc->context_lookup, index, ce) {
> > -		if (!intel_context_is_pinned(ce))
> > +		if (!kref_get_unless_zero(&ce->ref))
> >   			continue;
> > +		xa_unlock(&guc->context_lookup);
> > +
> > +		if (!intel_context_is_pinned(ce))
> > +			goto next;
> > +
> >   		if (intel_engine_is_virtual(ce->engine)) {
> >   			if (!(ce->engine->mask & engine->mask))
> > -				continue;
> > +				goto next;
> >   		} else {
> >   			if (ce->engine != engine)
> > -				continue;
> > +				goto next;
> >   		}
> > -		spin_lock_irqsave(&ce->guc_active.lock, flags);
> > +		spin_lock(&ce->guc_active.lock);
> >   		intel_engine_dump_active_requests(&ce->guc_active.requests,
> >   						  hung_rq, m);
> > -		spin_unlock_irqrestore(&ce->guc_active.lock, flags);
> > +		spin_unlock(&ce->guc_active.lock);
> > +
> > +next:
> > +		intel_context_put(ce);
> > +		xa_lock(&guc->context_lookup);
> >   	}
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> >   }
> >   void intel_guc_submission_print_info(struct intel_guc *guc,
> > @@ -2962,7 +3032,9 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
> >   {
> >   	struct intel_context *ce;
> >   	unsigned long index;
> > +	unsigned long flags;
> > +	xa_lock_irqsave(&guc->context_lookup, flags);
> >   	xa_for_each(&guc->context_lookup, index, ce) {
> >   		drm_printf(p, "GuC lrc descriptor %u:\n", ce->guc_id);
> >   		drm_printf(p, "\tHW Context Desc: 0x%08x\n", ce->lrc.lrca);
> > @@ -2981,6 +3053,7 @@ void intel_guc_submission_print_context_info(struct intel_guc *guc,
> >   		guc_log_context_priority(p, ce);
> >   	}
> > +	xa_unlock_irqrestore(&guc->context_lookup, flags);
> >   }
> >   static struct intel_context *
>