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Date:   Tue, 21 Jan 2020 10:16:33 +0100
From:   Borislav Petkov <bp@alien8.de>
To:     Marco Elver <elver@google.com>
Cc:     Qian Cai <cai@lca.pw>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH -next] x86/mm/pat: fix a data race in cpa_inc_4k_install
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 <CANpmjNPFRCRg9wnFwyJpZVg8Urb9HAdZ++e3xbh1LXPjgAs4kw@mail.gmail.com>
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On Tue, Jan 21, 2020 at 09:19:18AM +0100, Marco Elver wrote:
> As I said in my email that you also copied to the message, this is
> just a stats counter. For the general case, I think we reached
> consensus that such accesses should intentionally remain data races:
>   https://lore.kernel.org/linux-fsdevel/CAHk-=wg5CkOEF8DTez1Qu0XTEFw_oHhxN98bDnFqbY7HL5AB2g@mail.gmail.com/T/#u

Yap, I agree with Linus on the legibility aspect.

> Either you can use the data_race() macro, making this
> 'data_race(cpa_4k_install++)' -- this effectively documents the
> intentional data race --
> 
> or just blacklist the entire file by putting
>   KCSAN_SANITIZE_set_memory.o := n
> into the Makefile.
> 
> [ Note that there are 2 more ways to blacklist:
>   - __no_kcsan function attribute, for blacklisting entire functions.
>   - KCSAN_SANITIZE :=n in the Makefile, blacklisting all compilation
> units in the Makefile. ]

Do we have all those official methods how to make KCSAN happy,
documented somewhere?

> I leave it to you what makes more sense. I don't know if there are
> other data races lurking here, since cpa_4k_install is not the only
> stats counter.

In this particular case and if it were me, I'd prefer the __no_kcsan
function attribute because it is kept outside of the function body. But
I can't find __no_kcsan in current tip:

$ git grep __no_kcsan
.h:204:static __no_kcsan_or_inline bool constant_test_bit(long nr, const volatile unsigned long *addr)
include/linux/compiler.h:215:# define __no_kcsan_or_inline __no_sanitize_thread notrace __maybe_unused
include/linux/compiler.h:216:# define __no_sanitize_or_inline __no_kcsan_or_inline
include/linux/compiler.h:218:# define __no_kcsan_or_inline __always_inline
include/linux/compiler.h:225:static __no_kcsan_or_inline
include/linux/compiler.h:238:static __no_kcsan_or_inline

just this "glued together" thing __no_kcsan_or_inline.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette