Re: [Intel-gfx] [PATCH 05/16] drm/i915/gt: Move the breadcrumb to the signaler if completed upon cancel

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2020-11-24 16:19:15)
> 
> On 24/11/2020 11:42, Chris Wilson wrote:
> > If while we are cancelling the breadcrumb signaling, we find that the
> > request is already completed, move it to the irq signaler and let it be
> > signaled.
> > 
> > Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
> > ---
> >   drivers/gpu/drm/i915/gt/intel_breadcrumbs.c | 20 ++++++++++++++++----
> >   1 file changed, 16 insertions(+), 4 deletions(-)
> > 
> > diff --git a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > index a24cc1ff08a0..f5f6feed0fa6 100644
> > --- a/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > +++ b/drivers/gpu/drm/i915/gt/intel_breadcrumbs.c
> > @@ -363,6 +363,14 @@ void intel_breadcrumbs_free(struct intel_breadcrumbs *b)
> >       kfree(b);
> >   }
> >   
> > +static void irq_signal_request(struct i915_request *rq,
> > +                            struct intel_breadcrumbs *b)
> > +{
> > +     if (__signal_request(rq) &&
> > +         llist_add(&rq->signal_node, &b->signaled_requests))
> > +             irq_work_queue(&b->irq_work);
> > +}
> > +
> >   static void insert_breadcrumb(struct i915_request *rq)
> >   {
> >       struct intel_breadcrumbs *b = READ_ONCE(rq->engine)->breadcrumbs;
> > @@ -380,9 +388,7 @@ static void insert_breadcrumb(struct i915_request *rq)
> >        * its signal completion.
> >        */
> >       if (__request_completed(rq)) {
> > -             if (__signal_request(rq) &&
> > -                 llist_add(&rq->signal_node, &b->signaled_requests))
> > -                     irq_work_queue(&b->irq_work);
> > +             irq_signal_request(rq, b);
> >               return;
> >       }
> >   
> > @@ -453,6 +459,7 @@ bool i915_request_enable_breadcrumb(struct i915_request *rq)
> >   
> >   void i915_request_cancel_breadcrumb(struct i915_request *rq)
> >   {
> > +     struct intel_breadcrumbs *b = READ_ONCE(rq->engine)->breadcrumbs;
> >       struct intel_context *ce = rq->context;
> >       bool release;
> >   
> > @@ -461,11 +468,16 @@ void i915_request_cancel_breadcrumb(struct i915_request *rq)
> >   
> >       spin_lock(&ce->signal_lock);
> >       list_del_rcu(&rq->signal_link);
> > -     release = remove_signaling_context(rq->engine->breadcrumbs, ce);
> > +     release = remove_signaling_context(b, ce);
> >       spin_unlock(&ce->signal_lock);
> >       if (release)
> >               intel_context_put(ce);
> >   
> > +     if (__request_completed(rq)) {
> > +             irq_signal_request(rq, b);
> > +             return;
> 
> This is a bit unintuitive - irq_signal_request does things conditionally 
> based on the signaled flag, but here the return value is ignored and 
> reference kept regardless. Which makes me wonder how can the combo of 
> the two always dtrt. Because __request_completed is seqno based, which 
> can happen before setting the signaled flag. Like if retire races with 
> breadcrumbs. Am I missing something?

static void irq_signal_request()

Yes, the completion must happen before the signal bit is set, and there
is race on who sets the signal bit.

So if, and only if, __signal_request() is the first to set the signal
bit do we keep the reference to the request and enqueue it to execute the
callbacks in the irq-worker.

If the request is completed, but someone else has already signaled the
request, the reference is dropped.

static bool __signal_request(struct i915_request *rq)
{
        GEM_BUG_ON(test_bit(I915_FENCE_FLAG_SIGNAL, &rq->fence.flags));

        if (!__dma_fence_signal(&rq->fence)) {
                i915_request_put(rq);
                return false;
        }

        return true;
}

I see your point that the reference handling is not obvious. Worth
taking another pass over it to split the different paths into their
different ways so the ownership is not hidden away.
-Chris
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