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Date:   Wed, 27 Feb 2019 08:14:09 -0800
From:   Linus Torvalds <>
To:     Peter Zijlstra <>
Cc:     Ingo Molnar <>, Borislav Petkov <>,
        Thomas Gleixner <>,
        Andrew Lutomirski <>,
        Nadav Amit <>,
        Linux List Kernel Mailing <>,
        Nadav Amit <>
Subject: Re: [PATCH 1/5] x86/percpu: Differentiate this_cpu_{}() and __this_cpu_{}()

On Wed, Feb 27, 2019 at 2:16 AM Peter Zijlstra <> wrote:
> Nadav Amit reported that commit:
>   b59167ac7baf ("x86/percpu: Fix this_cpu_read()")
> added a bunch of constraints to all sorts of code; and while some of
> that was correct and desired, some of that seems superfluous.


I have the strong feeling that we should instead relax this_cpu_read()
again a bit.

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.

I would suggest that instead of making "this_cpu_read()" be asm
volatile, we mark it as potentially changing the memory location it is
touching - the same way the modify/write ops do.

That still means that the read will be forced (like READ_ONCE()), but
allows gcc a bit more flexibility in instruction scheduling, I think.

Trivial (but entirely untested) patch attached.

That said, I didn't actually check how it affects code generation.
Nadav, would you check the code sequences you originally noticed?


View attachment "patch.diff" of type "text/x-patch" (1348 bytes)

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