Re: [PATCH 2/2] perf_counter: optimize context switch betweenidentical inherited contexts

[Ingo Molnar]

* Peter Zijlstra <a.p.zijlstra@xxxxxxxxx> wrote:

> 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?

i think it should be an u64.

	Ingo
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/