On Wed, Feb 13, 2019 at 9:20 AM Will Deacon <will.dea...@arm.com> wrote:
>
> On Mon, Feb 11, 2019 at 02:34:31PM -0800, Linus Torvalds wrote:
> >
> > IOW, we should seriously just consider making the rule be that locking
> > will order mmio too. Because that's practically the rule anyway.
>
> I would /love/ to get rid of mmiowb() because I think it's both extremely
> difficult to use and also pretty much never needed. It reminds me a lot of
> smp_read_barrier_depends(), which we finally pushed into READ_ONCE for
> Alpha.
Right. Make as much of this implicit as we can.
At least as long as it's _reasonably_ cheap on all architectures that matter.
How expensive would it be on ARM? Does the normal acquire/release
already mean the IO in between is serialized?
> > Powerpc already does it. IO within a locked region will serialize with the
> > lock.
>
> I thought ia64 was the hold out here? Did they actually have machines that
> needed this in practice?
Note that even if mmiowb() is expensive (and I don't think that's
actually even the case on ia64), you can - and probably should - do
what PowerPC does.
Doing an IO barrier on PowerPC is insanely expensive, but they solve
that simply track the whole "have I done any IO" manually. It's not
even that expensive, it just uses a percpu flag.
(Admittedly, PowerPC makes it less obvious that it's a percpu variable
because it's actually in the special "paca" region that is like a
hyper-local percpu area).
> If so, I think we can either:
>
> (a) Add an mmiowb() to their spin_unlock() code, or
> (b) Remove ia64 altogether if nobody complains
>
> I know that Peter has been in favour of (b) for a while...
I don't think we're quite ready for (b), but see above: I don't think
adding mmiowb() to the ia64 spin unlock code is even all that
expensive.
Yeah, yeah, there's the SGI "SN" platform that apparently has a bug,
so because of that platform problem maybe it needs the more complex
"use a flag" model. But even the complex model isn't _hugely_
complex.
But we *could* first just do the mmiowb() unconditionally in the ia64
unlocking code, and then see if anybody notices?
Tony, comments? Are there any SGI SN machines around any more?
Linus
