Re: [PATCH RFC] perf_counter: Don't swap contexts containing locked mutex

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2009-05-29 at 10:06 +0200, Peter Zijlstra wrote:
> > @@ -919,7 +919,8 @@ static int context_equiv(struct perf_counter_context *ctx1,
> >                        struct perf_counter_context *ctx2)
> >  {
> >       return ctx1->parent_ctx && ctx1->parent_ctx == ctx2->parent_ctx
> > -             && ctx1->parent_gen == ctx2->parent_gen;
> > +             && ctx1->parent_gen == ctx2->parent_gen
> > +             && ctx1->parent_gen != ~0ull;
> >  }
> 
> There's a nasty surprise for people a few generations down the line. All
> of a sudden performance drops for a while for some unknown reason, and
> then its good again,.. how odd ;-)

OK, so I was joking, but my brain just came up with:

static void inc_generation(struct perf_counter_ctx *ctx)
{
	ctx->generation = ++ctx->generation & 0x7FFFFFFFFFFFFFFF;
}