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SubjectRe: Finegrained a/c/mtime was Re: Directory notification problem
Andi Kleen wrote:
> > Andi Kleen says we can ignore the risk; I disagree, as there are some
> > applications that cannot be trusted if the risk is plausible, and it can
> > be fixed easily.
>
> You're misquoting me badly. I said we can ignore the risk that two
> nanosecond resolution timestamps that get changed by two different cpus
> with out-of-sync cycle counter on a smp system and which are fast enough
> to free/aquire the inode lock in a smaller time than they're out of sync
> (= giving two file changes with the same ns timestamp) can be ignored.
> I implied on the systems that don't have a cycle counter and which use
> jiffie resolution gettimeofday it can be also ignored, because they're
> unlikely to be SMP and dying out too anyways.

Andi, sorry I misrepresented your statement.

I misread your original as saying that the risks due to SMP nanosecond
scale synchronisation problems can be ignored. Implied from that, that
the small risk of one SMP process modifying a file while another checks
the timestamp can be ignored. I misread this way because others have
suggested higher resolution solves the problem, and I believe it does not.

As you say above, multiple modifications within a single tick are not a
problem, do not have to be tracked, and therefore do not require SMP
sychronisation.

The SMP risk of missing a change after checking the timestamp is among
the risks I consider critical for an application which must not miss the
fact that a file has changed. I do not want us to repeat the mistake of
1 second at a smaller timescale.

cheers,
-- Jamie
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