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    SubjectRe: [RFC][PATCH] Catch xtime_nsec underflows and fix them
    Hi,

    On Wed, 3 Dec 2008, Zhang, Yanmin wrote:

    > > > When xtime_nsec underflows at the first time, xtime.tv_nsec becomes -1.
    > > > Later on when the second tick arrives, below statement in the while loop
    > > > clock->xtime_nsec += clock->xtime_interval;
    > > > will cause clock->xtime_nsec becomes positive again. So the second tick
    > > > appears a going-backward time.
    > >
    > > Yes, but only by 1nsec, so normally it wouldn't be noticable.
    > Not 1nsec. At the second tick, go back about 4294967296 nsec (2^32), about
    > 4 seconds. That matches the output of testing process.

    Above code is not the problem. The error appeared in do_vgettimeofday() as
    it only uses unsigned values, so the -1 became 0xffffffff in nsec and the
    clock jumped forward 4 seconds and a tick later 4 seconds back.

    bye, Roman
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