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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAHk-=wiv6dgJDj81wukpJCJBVhAVn-CYJdhj+o8JGN1LfRwb0g@mail.gmail.com>
Date:   Wed, 27 Feb 2019 08:14:09 -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 2:16 AM Peter Zijlstra <peterz@...radead.org> 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.

Hmm.

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?

                   Linus

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