linux-kernel - Re: [PATCH v2 00/21] Remove Mysterious Macro Intended to Obscure Weird Behaviours (mmiowb())

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Message-ID: <20190405160919.GA7857@fuggles.cambridge.arm.com>
Date:   Fri, 5 Apr 2019 17:09:19 +0100
From:   Will Deacon <will.deacon@....com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
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>,
        Michael Ellerman <mpe@...erman.id.au>,
        Arnd Bergmann <arnd@...db.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Palmer Dabbelt <palmer@...ive.com>,
        Daniel Lustig <dlustig@...dia.com>,
        David Howells <dhowells@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>,
        "Maciej W. Rozycki" <macro@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        Ingo Molnar <mingo@...nel.org>,
        Yoshinori Sato <ysato@...rs.sourceforge.jp>,
        Rich Felker <dalias@...c.org>, Tony Luck <tony.luck@...el.com>,
        Mikulas Patocka <mpatocka@...hat.com>,
        Akira Yokosawa <akiyks@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>
Subject: Re: [PATCH v2 00/21] Remove Mysterious Macro Intended to Obscure
 Weird Behaviours (mmiowb())

On Fri, Apr 05, 2019 at 05:55:37AM -1000, Linus Torvalds wrote:
> On Fri, Apr 5, 2019 at 3:59 AM Will Deacon <will.deacon@....com> wrote:
> >
> > I've also pushed this series out here:
> >
> >   git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux.git for-next/mmiowb
> >
> > and I would like to get it into -next once the first patch has been acked.
> 
> Ack on it all.

Thanks.

> With the afore-mentioned slight worry about non-spinlocked IO
> ordering, but I _think_ it's purely limited to ia64 and wmb() and
> friends should work elsewhere?
> 
> Or did I miss something? I think the ia64() mb/rmb/wmb stuff only
> works on normal memory on ia64.

I was worried about RISC-V, but actually their wmb() is "fence ow,ow"
which I think is stronger than their mmiowb() "fence o,w" implementation.

Everybody else should be fine with wmb() afaict, so if a driver writer
is smart enough to want this ordering outside of spinlocks, they can
do that for everybody apart from ia64.

Will