linux-kernel - Re: [RFC] Fast assurate clock readable from user space and NMI handler

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Message-Id: <1172605224.15970.43.camel@localhost>
Date:	Tue, 27 Feb 2007 11:40:24 -0800
From:	john stultz <johnstul@...ibm.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Daniel Walker <dwalker@...sta.com>, Ingo Molnar <mingo@...e.hu>,
	mbligh@...gle.com, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Fast assurate clock readable from user space and NMI
	handler

On Tue, 2007-02-27 at 14:04 -0500, Mathieu Desnoyers wrote:
> __get_nsec_offset : reads clock->cycle_last. Should be called with
> xtime_lock held. (ok so far, but see below)
> 
> change_clocksource
> clock->cycle_last = now; (non atomic 64 bits update. Not protected by
> any lock ?) -> this would race with __get_nsec_offset ?

Minor nit (it could probably use a comment fixup): its only called from
update_wall_time, which holds xtime_lock. 

> update_wall_time
> Called from timer interrupt. Holds xtime_lock and has a priority higher
> than other interrupts. Other clock->cycle_last protected by
> write_seqlock_irqsave.

-john

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