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References: <20200511204150.27858-18-will@kernel.org> <158929421358.390.2138794300247844367.tip-bot2@tip-bot2>
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From:   Marco Elver <elver@google.com>
Date:   Thu, 21 May 2020 00:30:39 +0200
Message-ID: <CANpmjNPsEMF8qq4qHoJEDb8mi211QzXODvnakh2fj3BOw+56MQ@mail.gmail.com>
Subject: Re: [tip: locking/kcsan] READ_ONCE: Use data_race() to avoid KCSAN instrumentation
To:     Borislav Petkov <bp@alien8.de>
Cc:     Will Deacon <will@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        LKML <linux-kernel@vger.kernel.org>,
        linux-tip-commits@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, x86 <x86@kernel.org>
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On Thu, 21 May 2020 at 00:17, Borislav Petkov <bp@alien8.de> wrote:
>
> Hi,
>
> On Tue, May 12, 2020 at 02:36:53PM -0000, tip-bot2 for Will Deacon wrote:
> > The following commit has been merged into the locking/kcsan branch of tip:
> >
> > Commit-ID:     cdd28ad2d8110099e43527e96d059c5639809680
> > Gitweb:        https://git.kernel.org/tip/cdd28ad2d8110099e43527e96d059c5639809680
> > Author:        Will Deacon <will@kernel.org>
> > AuthorDate:    Mon, 11 May 2020 21:41:49 +01:00
> > Committer:     Thomas Gleixner <tglx@linutronix.de>
> > CommitterDate: Tue, 12 May 2020 11:04:17 +02:00
> >
> > READ_ONCE: Use data_race() to avoid KCSAN instrumentation
> >
> > Rather then open-code the disabling/enabling of KCSAN across the guts of
> > {READ,WRITE}_ONCE(), defer to the data_race() macro instead.
> >
> > Signed-off-by: Will Deacon <will@kernel.org>
> > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> > Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> > Cc: Marco Elver <elver@google.com>
> > Link: https://lkml.kernel.org/r/20200511204150.27858-18-will@kernel.org
>
> so this commit causes a kernel build slowdown depending on the .config
> of between 50% and over 100%. I just bisected locking/kcsan and got
>
> NOT_OK: cdd28ad2d811 READ_ONCE: Use data_race() to avoid KCSAN instrumentation
> OK:     88f1be32068d kcsan: Rework data_race() so that it can be used by READ_ONCE()
>
> with a simple:
>
> $ git clean -dqfx && mk defconfig
> $ time make -j<NUM_CORES+1>
>
> I'm not even booting the kernels - simply checking out the above commits
> and building the target kernels. I.e., something in that commit is
> making gcc go nuts in the compilation phases.

This should be fixed when the series that includes this commit is applied:
https://lore.kernel.org/lkml/20200515150338.190344-9-elver@google.com/

Thanks,
-- Marco