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    SubjectRe: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]
    On Fri, 11 Jul 2014, Mathieu Desnoyers wrote:

    > ----- Original Message -----
    > > From: "Thomas Gleixner" <tglx@linutronix.de>
    > > To: "LKML" <linux-kernel@vger.kernel.org>
    > > Cc: "John Stultz" <john.stultz@linaro.org>, "Peter Zijlstra" <peterz@infradead.org>, "Steven Rostedt"
    > > <rostedt@goodmis.org>, "Mathieu Desnoyers" <mathieu.desnoyers@efficios.com>
    > > Sent: Friday, July 11, 2014 9:45:19 AM
    > > Subject: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]
    > > On the update side:
    > >
    > > tkf->seq++;
    > > smp_wmb();
    > > update(tkf->base[0], tk;
    >
    > missing ")"

    :)

    > Any reason why the updater wouldn't do:
    >
    > tkf->seq++;
    > smp_wmb();
    > update(tkf->base[1 - (tkf->seq & 0x01)], tk);
    >
    > instead of updating both array entries each time ?

    base[0]; <- Current active
    seq++; -> Makes base[1] the active one for readers
    update(base[0]);

    So readers are always one update cycle behind. Probably not an issue
    most of the time, but think about fast wrapping clocksources and
    NOHZ....

    Thanks,

    tglx




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