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    SubjectRe: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

    * Peter Zijlstra <a.p.zijlstra@chello.nl> 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


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