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

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Message-ID: <18969.58137.356360.144339@cargo.ozlabs.ibm.com>
Date:	Mon, 25 May 2009 10:15:21 +1000
From:	Paul Mackerras <paulus@...ba.org>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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

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.

Paul.
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