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Message-ID: <53EC176D.6080201@redhat.com> Date: Wed, 13 Aug 2014 21:57:01 -0400 From: Rik van Riel <riel@...hat.com> To: Frederic Weisbecker <fweisbec@...il.com> CC: Oleg Nesterov <oleg@...hat.com>, 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 -----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: >> --- a/kernel/time/posix-cpu-timers.c +++ >> b/kernel/time/posix-cpu-timers.c @@ -272,22 +272,8 @@ static int >> posix_cpu_clock_get_task(struct task_struct *tsk, if >> (same_thread_group(tsk, current)) err = >> cpu_clock_sample(which_clock, tsk, &rtn); } else { - unsigned >> long flags; - struct sighand_struct *sighand; - - /* - * >> while_each_thread() is not yet entirely RCU safe, - * keep >> locking the group while sampling process - * clock for now. - >> */ - sighand = lock_task_sighand(tsk, &flags); - if (!sighand) >> - return err; - if (tsk == current || >> thread_group_leader(tsk)) err = >> cpu_clock_sample_group(which_clock, tsk, &rtn); - - >> unlock_task_sighand(tsk, &flags); } > > 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? - -- All rights reversed -----BEGIN PGP SIGNATURE----- Version: GnuPG v1 iQEcBAEBAgAGBQJT7BdtAAoJEM553pKExN6DngEH/1CJuBb6xij08AoZNQuW4WNQ geKakADsYTz8FmutbGi+lJEHNKAMZQ5wYbyFNczPAX/fVJsOlC92Qtfwy5z/VupN QzlRHh79ZJR5/6xGddlu/8LjGrMIXwKqShIeKtTzoENS+rxA82l42zoXTagal4yX Peb5/Q7cBMg9pFZzUMITEJssQhDtyTN1rXiU5IsEG54PhDbSgFk7HK1cO46tRefb x1WbUKZKUViVKnoKYhJqd6FDSWuPtL5EpebvMVj9TZ29JBQTMDGx8saUezjuY0YL STAv/wqigmbbcNnloJpr3gO1iJMkknv3jHk6Bfv1Dil8Um1D3mBAAKFK+Ov8Rk0= =kU1O -----END PGP SIGNATURE----- -- 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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