linux-kernel - Re: [patch 1/2] x86_64 page fault NMI-safe

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Date:	Fri, 06 Aug 2010 11:51:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Steven Rostedt <rostedt@...tedt.homelinux.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@....de>, Li Zefan <lizf@...fujitsu.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Johannes Berg <johannes.berg@...el.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Tom Zanussi <tzanussi@...il.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Andi Kleen <andi@...stfloor.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"Frank Ch. Eigler" <fche@...hat.com>, Tejun Heo <htejun@...il.com>
Subject: Re: [patch 1/2] x86_64 page fault NMI-safe

On Thu, 2010-08-05 at 21:49 -0400, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Wed, 2010-08-04 at 10:45 -0400, Mathieu Desnoyers wrote:
> > 
> > > How do you plan to read the data concurrently with the writer overwriting the
> > > data while you are reading it without corruption ?
> > 
> > I don't consider reading while writing (in overwrite mode) a valid case.
> > 
> > If you want to use overwrite, stop the writer before reading it.
> 
> How inconvenient. It happens that the relatively large group of users I am
> working for do care for this use-case. They cannot afford to stop tracing as
> soon as they hit "one bug". This "bug" could be a simple odd scenario that they
> want to snapshot, but in all cases they want tracing to continue.

Snapshot is fine, just swivel the whole buffer.
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