Re: [PATCH] CPU time clock support in clock_* syscalls

[Nick Piggin]

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.
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/