linux-kernel - Re: [PATCH] sched_clock: Prevent callers from seeing half-updated data

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Date:	Tue, 18 Feb 2014 21:53:59 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	John Stultz <john.stultz@...aro.org>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Stephen Boyd <sboyd@...eaurora.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	Will Deacon <will.deacon@....com>,
	Josh Cartwright <joshc@...eaurora.org>
Subject: Re: [PATCH] sched_clock: Prevent callers from seeing half-updated
 data

On Tue, 18 Feb 2014, John Stultz wrote:
> On Tue, Feb 18, 2014 at 12:20 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> > On Mon, 17 Feb 2014, John Stultz wrote:
> >
> >> From: Stephen Boyd <sboyd@...eaurora.org>
> >>
> >> The generic sched_clock registration function was previously
> >> done lockless, due to the fact that it was expected to be called
> >> only once. However, now there are systems that may register
> >> multiple sched_clock sources, for which the lack of locking has
> >> casued problems:
> >>
> >> If two sched_clock sources are registered we may end up in a
> >> situation where a call to sched_clock() may be accessing the
> >> epoch cycle count for the old counter and the cycle count for the
> >> new counter. This can lead to confusing results where
> >> sched_clock() values jump and then are reset to 0 (due to the way
> >> the registration function forces the epoch_ns to be 0).
> >>
> >> Fix this by reorganizing the registration function to hold the
> >> seqlock for as short a time as possible while we update the
> >> clock_data structure for a new counter. We also put any
> >> accumulated time into epoch_ns instead of resetting the time to
> >> 0 so that the clock doesn't reset after each successful
> >> registration.
> >>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >
> > Peter ???
> 
> So this is the generic sched_clock work, which Peter really hasn't had
> much involvement with yet (mostly because its not yet generic enough
> to work with more then a few arches). But I included him in the CC
> since I think it would be good to have him following along.

Fair enough. I'll pick it up.

Thanks,

	tglx
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