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Date:   Wed, 27 Feb 2019 11:19:25 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Will Deacon <will.deacon@....com>,
        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>,
        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>
Subject: Re: [RFC PATCH 01/20] asm-generic/mmiowb: Add generic implementation
 of mmiowb() tracking

On Tue, Feb 26, 2019 at 11:02:50AM -0800, Linus Torvalds wrote:
> On Tue, Feb 26, 2019 at 10:33 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Arguably we could fix that for __this_cpu_xchg(), which isn't IRQ-safe.
> 
> Yeah, I guess x86 _should_ really do __this_cpu_xchg() as just a
> read-write pair.

See the patches I just send.

> In general, a read-write pair is probably always the right thing to
> do, and the only reason we can't just do it in an
> architecture-independent way is that we'd want to avoid doing the
> address generation twice (for architectures where that is an issue).

The generic code has this right, see
include/asm-generic/percpu.h:raw_cpu_generic_xchg().

That is used in the majority of the architectures. With the patch I just
send, x86 will use two gs prefixed movs.

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