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Date:   Thu, 21 May 2020 00:17:12 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Will Deacon <will@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Marco Elver <elver@google.com>, x86 <x86@kernel.org>
Subject: Re: [tip: locking/kcsan] READ_ONCE: Use data_race() to avoid KCSAN
 instrumentation
Message-ID: <20200520221712.GA21166@zn.tnic>
References: <20200511204150.27858-18-will@kernel.org>
 <158929421358.390.2138794300247844367.tip-bot2@tip-bot2>
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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.

-- 
Regards/Gruss,
    Boris.

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