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

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Message-Id: <1242981131.26820.620.camel@twins>
Date:	Fri, 22 May 2009 10:32:11 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
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 Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:

> The equivalence of contexts is detected by keeping a pointer in
> each cloned context pointing to the context it was cloned from.
> To cope with the situation where a context is changed by adding
> or removing counters after it has been cloned, we also keep a
> generation number on each context which is incremented every time
> a context is changed.  When a context is cloned we take a copy
> of the parent's generation number, and two cloned contexts are
> equivalent only if they have the same parent and the same
> generation number.  In order that the parent context pointer
> remains valid (and is not reused), we increment the parent
> context's reference count for each context cloned from it.

> +	u32			generation;

Suppose someone writes a malicious proglet that inherits the counters,
puts the child to sleep, does 2^32 mods on the counter set, and then
wakes up the child.

Would that merely corrupt the results, or make the kernel explode?

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