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

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Message-ID: <CAHk-=wi_0GUBgi3guWqUzT3gZOAC+iEyZcSLdw0=_9xWJEwHZg@mail.gmail.com>
Date:   Wed, 27 Feb 2019 09:17:42 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        Nadav Amit <namit@...are.com>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Nadav Amit <nadav.amit@...il.com>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()

On Wed, Feb 27, 2019 at 8:48 AM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Wed, Feb 27, 2019 at 08:14:09AM -0800, Linus Torvalds wrote:
> > In particular, making it "asm volatile" really is a big hammer
> > approach. It's worth noting that the *other* this_cpu_xyz ops don't
> > even do that.
>
> Right, this patch 'fixes' that :-)

Yeah, but I do hate seeing these patches that randomly add double
underscore versions just because the regular one is so bad.

So I'd much rather improve the regular this_cpu_read() instead, and
hope that we don't need to have this kind of big difference between
that and the double-underscore version.

I'm not convinced that the "asm volatile" is right on the _other_ ops
either. You added them to cmpxchg and xadd too, and it's not obvious
that they should have them. They have the "memory" clobber to order
them wrt actual memory ops, why are they now "asm volatile"?

So I don't really like this patch that randomly adds volatile to
things, and then removes it from one special case that I don't think
should have had it in the first place.

It all seems pretty ad-hoc, and we already _know_ that "asm volatile" is bad.

                      Linus