linux-kernel - Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER EFFECTS" section

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Date:   Wed, 13 Feb 2019 10:27:09 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Will Deacon <will.deacon@....com>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        Tony Luck <tony.luck@...el.com>
Subject: Re: [RFC PATCH] docs/memory-barriers.txt: Rewrite "KERNEL I/O BARRIER
 EFFECTS" section

On Wed, Feb 13, 2019 at 9:20 AM Will Deacon <will.deacon@....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