linux-kernel - Re: [PATCH 4/2] perf_counter: Fix swcounter context invariance

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Message-Id: <1250151778.10001.67.camel@twins>
Date:	Thu, 13 Aug 2009 10:22:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Paul Mackerras <paulus@...ba.org>,
	stephane eranian <eranian@...glemail.com>,
	Corey J Ashford <cjashfor@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/2] perf_counter: Fix swcounter context invariance

On Thu, 2009-08-13 at 10:05 +0200, Frederic Weisbecker wrote:
> On Thu, Aug 13, 2009 at 09:51:55AM +0200, Peter Zijlstra wrote:
> > Not really related to this topic, but it needs posting anyway.
> > 
> > ---
> > 
> > Subject: perf_counter: Fix swcounter context invariance
> > From: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> > Date: Fri Aug 07 13:29:13 CEST 2009
> > 
> > perf_swcounter_is_counting() uses a lock, which means we cannot use
> > swcounters from NMI or when holding that particular lock, this is
> > unintended.
> > 
> > The below removes the lock, this opens up race window, but not worse
> > than the swcounters already experience due to RCU traversal of the
> > context in perf_swcounter_ctx_event().
> > 
> > Cc: Paul Mackerras <paulus@...ba.org>
> > Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> 
> 
> 
> As a side effect, it's possible this also fixes the hard lockups
> while opening a lockdep tracepoint counter.

It will -- not a side-effect at all, intended consequence :-)
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