On 08/14, Frederic Weisbecker wrote: > > 2014-08-14 3:57 GMT+02:00 Rik van Riel <r...@redhat.com>: > > -----BEGIN PGP SIGNED MESSAGE----- > > Hash: SHA1 > > > > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
