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    SubjectRe: [tip: locking/kcsan] READ_ONCE: Use data_race() to avoid KCSAN instrumentation
    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

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