linux-kernel - Re: [PATCH -next] x86/mm/pat: silence a data race in cpa_4k

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Message-ID: <20200122084604.GP14914@hirez.programming.kicks-ass.net>
Date:   Wed, 22 Jan 2020 09:46:04 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Marco Elver <elver@...gle.com>, Qian Cai <cai@....pw>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] x86/mm/pat: silence a data race in cpa_4k_install

On Tue, Jan 21, 2020 at 04:45:28PM +0100, Borislav Petkov wrote:
> On Tue, Jan 21, 2020 at 04:36:49PM +0100, Marco Elver wrote:
> > Isn't the intent "x86/mm/pat: Mark intentional data race" ?  The fact
> > that KCSAN no longer shows the warning is a side-effect.  At least
> > that's how I see it.
> 
> Perhaps because you've been dealing with KCSAN for so long. :-)
> 
> The main angle here, IMO, is that this "fix" is being done solely for
> KCSAN. Or is there another reason to "fix" intentional data races? At
> least I don't see one. And the text says

Documentation. It is a clear and concise marker of intent. Unintended
data races are bad.

Also, we've been adding annotations to the kernel source forever,
sparse, lockdep, etc.. now KCSAN. All we have to do is make sure they're
minimally invasive, and in that regard the date_race() marker is spot on
IMO.