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

[Thomas Gleixner]

On Fri, 11 Jul 2014, Mathieu Desnoyers wrote:

> ----- Original Message -----
> > From: "Thomas Gleixner" <tglx@xxxxxxxxxxxxx>
> > To: "LKML" <linux-kernel@xxxxxxxxxxxxxxx>
> > Cc: "John Stultz" <john.stultz@xxxxxxxxxx>, "Peter Zijlstra" <peterz@xxxxxxxxxxxxx>, "Steven Rostedt"
> > <rostedt@xxxxxxxxxxx>, "Mathieu Desnoyers" <mathieu.desnoyers@xxxxxxxxxxxx>
> > 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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