linux-kernel - Re: [PATCH v2 17/21] drivers: Remove explicit invocations of mmiowb()

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Date:   Fri, 5 Apr 2019 05:50:32 -1000
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>,
        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 17/21] drivers: Remove explicit invocations of mmiowb()

On Fri, Apr 5, 2019 at 4:01 AM Will Deacon <will.deacon@....com> wrote:
>
> mmiowb() is now implied by spin_unlock() on architectures that require
> it, so there is no reason to call it from driver code. This patch was
> generated using coccinelle:
>
>         @mmiowb@
>         @@
>         - mmiowb();

So I love the patch series, and think we should just do it, but I do
wonder if some of the drivers involved end up relying on memory
ordering things (store_release -> load_aquire) and IO ordering rather
than using locking...

Wouldn't such use now be broken on ia64 SN platforms? Do we care?

So it might be worth noting that a lot of the mmiowb()s here weren't
paired with spin_unlock?

                    Linus