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

Ingo Molnar
2009-05-22T08:57:42Z

* 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