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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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