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Message-ID: <20140814143902.GA29052@redhat.com> Date: Thu, 14 Aug 2014 16:39:02 +0200 From: Oleg Nesterov <oleg@...hat.com> To: Frederic Weisbecker <fweisbec@...il.com> Cc: Rik van Riel <riel@...hat.com>, LKML <linux-kernel@...r.kernel.org>, Peter Zijlstra <peterz@...radead.org>, Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>, Frank Mayhar <fmayhar@...gle.com>, Frederic Weisbecker <fweisbec@...hat.com>, Andrew Morton <akpm@...ux-foundation.org>, Sanjay Rao <srao@...hat.com>, Larry Woodman <lwoodman@...hat.com> Subject: Re: [PATCH RFC] time,signal: protect resource use statistics with seqlock On 08/14, Frederic Weisbecker wrote: > > 2014-08-14 3:57 GMT+02:00 Rik van Riel <riel@...hat.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 majordomo@...r.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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