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    SubjectRe: [Patch] Re: Nasty suprise with uptime
    On Oct 31, 2001  19:16 +0100, Tim Schmielau wrote:
    > The idea was that all drivers that use the 32 bit jiffies counter have to
    > be aware of the wraparound anyways, and won't see a difference.

    Agreed. I also like the change that you initialize jiffies to a pre-wrap
    value, so the jiffies wrap bugs can more easily be found/fixed.

    > The race only happens for 64 bit accesses to jiffies, but hey, without
    > the patch these values come out wrong _every_ time, so I believed a
    > tiny window for a single wrong display of uptime every 497.1 days to be
    > acceptable.

    I would say that the race is so rare that it should not be handled, especially
    since it adds extra code in the timer interrupt.

    > + /* We need to make sure jiffies_high does not change while
    > + * reading jiffies and jiffies_high */
    > + do {
    > + jiffies_high_tmp = jiffies_high_shadow;
    > + barrier();
    > + jiffies_tmp = jiffies;
    > + barrier();
    > + } while (jiffies_high != jiffies_high_tmp);

    Maybe this could be condensed into a macro/inline, so that people don't
    screw it up (and it looks cleaner). Like get_jiffies64() or so, for
    those few places that really care about the full value and can't stand
    a miniscule chance of a race (i.e. uptime output is not a candidate).

    Cheers, Andreas
    Andreas Dilger

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