linux-kernel - Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[

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Date:	Sat, 12 Jul 2014 10:11:17 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [patch 54/55] timekeeping: Provide fast and NMI safe access to
 CLOCK_MONOTONIC[_RAW]

On Fri, 11 Jul 2014, Mathieu Desnoyers wrote:

> ----- Original Message -----
> > From: "Thomas Gleixner" <tglx@...utronix.de>
> > To: "LKML" <linux-kernel@...r.kernel.org>
> > Cc: "John Stultz" <john.stultz@...aro.org>, "Peter Zijlstra" <peterz@...radead.org>, "Steven Rostedt"
> > <rostedt@...dmis.org>, "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> > Sent: Friday, July 11, 2014 9:45:19 AM
> > Subject: [patch 54/55] timekeeping: Provide fast and NMI safe access to CLOCK_MONOTONIC[_RAW]
> > On the update side:
> > 
> >   tkf->seq++;
> >   smp_wmb();
> >   update(tkf->base[0], tk;
> 
> missing ")"

:)
 
> Any reason why the updater wouldn't do:
> 
> tkf->seq++;
> smp_wmb();
> update(tkf->base[1 - (tkf->seq & 0x01)], tk); 
> 
> instead of updating both array entries each time ?

  base[0];   	      <- Current active
  seq++;	      -> Makes base[1] the active one for readers
  update(base[0]);

So readers are always one update cycle behind. Probably not an issue
most of the time, but think about fast wrapping clocksources and
NOHZ....

Thanks,

	tglx


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