linux-kernel - Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu

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Date:   Fri, 8 Mar 2019 14:35:10 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Nadav Amit <namit@...are.com>, Ingo Molnar <mingo@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and
 __this_cpu_{}()

On Wed, Feb 27, 2019 at 11:41:31AM -0800, Linus Torvalds wrote:
> On Wed, Feb 27, 2019 at 9:57 AM Nadav Amit <namit@...are.com> wrote:
> >
> > I’ll have a look at some specific function assembly, but overall, the “+m”
> > approach might prevent even more code optimizations than the “volatile” one.
> 
> Ok, that being the case, let's forget that patch.
> 
> I still wonder about the added volatiles to the xadd/cmpxchg cases,
> which already had the "memory" clobber which should make the volatile
> immaterial..

That was mostly me being OCD style consistent; but also notice that the
atomic ops also often have volatile even though they have a memory
clobber.