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SubjectRe: [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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