Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock

Oleg Nesterov
2014-08-14T14:41:35Z

On 08/14, Frederic Weisbecker wrote:
>
> 2014-08-14 3:57 GMT+02:00 Rik van Riel <riel@redhat.com>:
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> > On 08/13/2014 08:43 PM, Frederic Weisbecker wrote:
> >> On Wed, Aug 13, 2014 at 05:03:24PM -0400, Rik van Riel wrote:
> >>
> >> I'm worried about such lockless solution based on RCU or read
> >> seqcount because we lose the guarantee that an update is
> >> immediately visible by all subsequent readers.
> >>
> >> Say CPU 0 updates the thread time and both CPU 1 and CPU 2 right
> >> after that call clock_gettime(), with the spinlock we were
> >> guaranteed to see the new update. Now with a pure seqlock read
> >> approach, we guarantee a read sequence coherency but we don't
> >> guarantee the freshest update result.
> >>
> >> So that looks like a source of non monotonic results.
> >
> > Which update are you worried about, specifically?
> >
> > The seq_write_lock to update the usage stat in p->signal will lock out
> > the seqlock read side used to check those results.
> >
> > Is there another kind of thing read by cpu_clock_sample_group that you
> > believe is not excluded by the seq_lock?
>
> I mean the read side doesn't use a lock with seqlocks. It's only made
> of barriers and sequence numbers to ensure the reader doesn't read
> some half-complete update. But other than that it can as well see the
> update n - 1 since barriers don't enforce latest results.

Yes, sure, read_seqcount_begin/read_seqcount_retry "right after"
write_seqcount_begin-update-write_seqcount_begin can miss "update" part
along with ->sequence modifications.

But I still can't understand how this can lead to non-monotonic results,
could you spell?

Oleg.