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    SubjectRe: [PATCH] CPU time clock support in clock_* syscalls
    Roland McGrath wrote:
    >>It seemed like a syscall could read the values from a task currently
    >>running on another CPU. If not, great.
    >
    >
    > Indeed it can. And yes, there is no locking against updates for this. For
    > sched_time on 32-bit platforms, there is the possibility it could be read
    > during an update and give a bogus value if the high half is updated before
    > the low half. Since there are no guarantees about accuracy, period, I
    > decided not to worry about such an anomaly. Perhaps it would be better to
    > do something about this, but AFAIK nothing perfect can be done without
    > adding more words to task_struct (e.g. seqcount). I don't know if the
    > nature of SMP cache behavior makes something like:
    >
    > do {
    > sample = p->sched_time;
    > } while (p->sched_time != sample);
    >
    > sufficient. That would certainly be easy to do.
    >

    I don't think that will be quite sufficient.

    A seqcount would probably be the way to go.
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