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

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Message-ID: <CALAqxLX4k4JcUGzVdv68w+4v0fg34G_qBiBhH07kyTssQ1jE1Q@mail.gmail.com>
Date:	Tue, 18 Feb 2014 12:32:45 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Thomas Gleixner <tglx@...utronix.de>
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, 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.

thanks
-john
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