Messages in this thread | | | Date | Wed, 06 Oct 2004 10:51:00 +1000 | From | Nick Piggin <> | Subject | Re: [PATCH] CPU time clock support in clock_* syscalls |
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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@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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