Re: MMIO and gcc re-ordering issue

Jesse Barnes
2008-06-11T16:07:11Z

On Tuesday, June 10, 2008 8:29 pm Nick Piggin wrote:
> On Wednesday 11 June 2008 05:19, Jesse Barnes wrote:
> > On Tuesday, June 10, 2008 12:05 pm Roland Dreier wrote:
> > > > me too. That's the whole basis for readX_relaxed() and its cohorts:
> > > > we make our weirdest machines (like altix) conform to the x86 norm.
> > > > Then where it really kills us we introduce additional semantics to
> > > > selected drivers that enable us to recover I/O speed on the abnormal
> > > > platforms.
> > >
> > > Except as I pointed out before, Altix doesn't conform to the norm and
> > > many (most?) drivers are missing mmiowb()s that are needed for Altix.
> > > Just no one has plugged most devices into an Altix (or haven't stressed
> > > the driver in a way that exposes problems of IO ordering between CPUs).
> > >
> > > It would be a great thing to use the powerpc trick of setting a flag
> > > that is tested by spin_unlock()/mutex_unlock() and automatically doing
> > > the mmiowb() if needed, and then killing off mmiowb() entirely.
> >
> > Yeah I think that's what Nick's guidelines would guarantee. And Jes is
> > already working on the spin_unlock change you mentioned, so mmiowb()
> > should be history soon (in name only, assuming Nick also introduces the
> > I/O barriers he talked about for ordering the looser accessors it would
> > still be there but would be called io_wmb or something).
>
> Exactly, yes. I guess everybody has had good intentions here, but
> as noticed, what is lacking is coordination and documentation.
>
> You mention strong ordering WRT spin_unlock, which suggests that
> you would prefer to take option #2 (the current powerpc one): io/io
> is ordered and io is contained inside spinlocks, but io/cacheable
> in general is not ordered.

I was thinking it would be good for the weaker accessors, but now that I think
about it you could just use the new io_* barrier functions.

I didn't mean to imply that I wasn't in favor of the io/cacheable ordering as
well.

> For any high performance drivers that are well maintained (ie. the
> ones where slowdown might be noticed), everyone should have a pretty
> good handle on memory ordering requirements, so it shouldn't take
> long to go through and convert them to relaxed accessors.

Yep. Thanks for working on this, Nick, it's definitely a good thing that
you're taking control of it. :)

Thanks,
Jesse