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

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2009-05-25 at 12:38 +0200, Peter Zijlstra wrote:
> On Mon, 2009-05-25 at 10:15 +1000, Paul Mackerras wrote:
> > Peter Zijlstra writes:
> > 
> > > Ingo just pointed out that there is nothing there to close the race with
> > > attaching a counter.
> > > 
> > > That is, you could end up attaching your counter to the wrong task.
> > 
> > Good point.  Doing the unclone in find_get_context would be a start,
> > but the locking could get tricky (in fact we don't have any way to
> > remove an inherited counter from a context, so we only have to worry
> > about counters being attached).  I'll work out a solution after I have
> > digested your recent batch of patches.
> 
> I'm currently staring at something like the below, trying to find races
> etc.. ;-)
> 
> attach vs destroy vs flip

Hmm, it appears we only unclone ctx on remove, not attach.. how odd..