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

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Message-Id: <1172571535.5517.222.camel@imap.mvista.com>
Date:	Tue, 27 Feb 2007 02:18:55 -0800
From:	Daniel Walker <dwalker@...sta.com>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Ingo Molnar <mingo@...e.hu>, mbligh@...gle.com,
	linux-kernel@...r.kernel.org, johnstul@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC] Fast assurate clock readable from user space and NMI
	handler

On Tue, 2007-02-27 at 02:38 -0500, Mathieu Desnoyers wrote:

> 
> I am concerned about the automatic fallback to the PIT when no other
> clock source is available. A clocksource read would be atomic when TSC
> or HPET are available, but would fall back on PIT otherwise. There
> should be some way to specify that a caller is only interested in atomic
> clock sources (if none are available, the call should simply return an
> error, or 0).
> 
> I still think that an RCU style update mechanism would be a good way  to
> fix the current clocksource read issue. Another, slower and non NMI
> safe way to do this would be with a read seqlock and with IRQ disabling.

I'm not sure what you mean by using the RCU, but the pit clocksource
does disable interrupts with a spin_lock_irqsave().

Daniel

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