linux-kernel - Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

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Message-Id: <1243248603.26820.652.camel@twins>
Date:	Mon, 25 May 2009 12:50:03 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between
 identical inherited contexts

On Mon, 2009-05-25 at 12:38 +0200, Peter Zijlstra wrote:
> On Mon, 2009-05-25 at 10:15 +1000, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > Ingo just pointed out that there is nothing there to close the race with
> > > attaching a counter.
> > > 
> > > That is, you could end up attaching your counter to the wrong task.
> > 
> > Good point.  Doing the unclone in find_get_context would be a start,
> > but the locking could get tricky (in fact we don't have any way to
> > remove an inherited counter from a context, so we only have to worry
> > about counters being attached).  I'll work out a solution after I have
> > digested your recent batch of patches.
> 
> I'm currently staring at something like the below, trying to find races
> etc.. ;-)
> 
> attach vs destroy vs flip

Hmm, it appears we only unclone ctx on remove, not attach.. how odd..
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