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Subject: Re: [PATCH -next] x86/mm/pat: silence a data race in cpa_4k_install
From:   Qian Cai <cai@lca.pw>
In-Reply-To: <20200121154528.GK7808@zn.tnic>
Date:   Tue, 21 Jan 2020 15:21:35 -0500
Cc:     Marco Elver <elver@google.com>,
        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>
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To:     Borislav Petkov <bp@alien8.de>
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> On Jan 21, 2020, at 10:45 AM, Borislav Petkov <bp@alien8.de> 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
> 
> "This will generate a lot of noise on a debug kernel with
> debug_pagealloc with KCSAN enabled which could render the system
> unusable."
> 
> So yes, I think it should say something about making KCSAN happy.
> 
> Oh, and while at it I'd prefer it if it did the __no_kcsan function
> annotation instead of the data_race() thing.

Actually "__no_kcsan" does not work because I have
CONFIG_OPTIMIZE_INLINING=y (GCC 8.3.1) here, so it has to be,

diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
index 20823392f4f2..fabbf8a33b7f 100644
--- a/arch/x86/mm/pat/set_memory.c
+++ b/arch/x86/mm/pat/set_memory.c
@@ -126,7 +126,7 @@ static inline void cpa_inc_2m_checked(void)
        cpa_2m_checked++;
 }
 
-static inline void cpa_inc_4k_install(void)
+static inline void __no_kcsan_or_inline cpa_inc_4k_install(void)
 {
        cpa_4k_install++;
 }

Are you fine with it or data_race() looks better?