linux-kernel - Re: [RFC PATCH 16/22 -v2] add get_monotonic

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Message-ID: <Pine.LNX.4.58.0801162150490.20257@gandalf.stny.rr.com>
Date:	Wed, 16 Jan 2008 21:51:57 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Christoph Hellwig <hch@...radead.org>,
	Gregory Haskins <ghaskins@...ell.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tim Bird <tim.bird@...sony.com>,
	Sam Ravnborg <sam@...nborg.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	Steven Rostedt <srostedt@...hat.com>,
	Paul Mackerras <paulus@...ba.org>,
	Daniel Walker <dwalker@...sta.com>
Subject: Re: [RFC PATCH 16/22 -v2] add get_monotonic_cycles


On Wed, 16 Jan 2008, Mathieu Desnoyers wrote:
> >
> > Yep. clocksource_get_cycles() ended up not being as useful as an helper
> > function (I was hoping the arch vsyscall implementations could use it,
> > but they've done too much optimization - although that may reflect a
> > need up the chain to the clocksource structure).
> >
>
> The problem with vsyscall is that we will have a hard time disabling
> preemption :( Therefore, insuring that the read of the data is done in a
> timely manner is hard to do.

You'll have more than a hard time disabling preemption for vsyscall. We'll
need to come up with a better solution then. vsyscall can not modify any
kernel memory, nor can it disable preemption.

-- Steve

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