Quoting Tvrtko Ursulin (2020-02-25 19:07:43)
>
> On 24/02/2020 09:59, Chris Wilson wrote:
> > -int i915_active_wait(struct i915_active *ref)
> > +static int flush_barrier(struct active_node *it)
> > {
> > - struct active_node *it, *n;
> > - int err = 0;
> > + struct intel_engine_cs *engine;
> >
> > - might_sleep();
> > + if (!is_barrier(&it->base))
> > + return 0;
> >
> > - if (!i915_active_acquire_if_busy(ref))
> > + engine = __barrier_to_engine(it);
> > + smp_rmb(); /* serialise with add_active_barriers */
> > + if (!is_barrier(&it->base))
> > return 0;
>
> What is the purpose of the first !is_barrier check? Just to kind of look
> better by not calling __bariier_to_engine on the wrong thing?
Yeah, and that smp_rmb() is on the expensive side (enough to justify a
branch). If I was confident I would mark up that first !is_barrier() with
likely(). Hmm, does the kernel still have the infrastructure to warn
those annotations are wrong?
-Chris
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