On Fri, Dec 11, 2015 at 04:02:36PM +0100, Peter Zijlstra wrote:
> On Fri, Dec 11, 2015 at 03:36:36PM +0200, Alexander Shishkin wrote:
> > +static int perf_event_itrace_filters_setup(struct perf_event *event)
> > +{
> > + int ret;
> > +
> > + /*
> > + * We can't use event_function_call() here, because that would
> > + * require ctx::mutex, but one of our callers is called with
> > + * mm::mmap_sem down, which would cause an inversion, see bullet
> > + * (2) in put_event().
> > + */
> > + do {
> > + if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE) {
> > + ret = event->pmu->itrace_filter_setup(event);
> > + break;
>
> So this is tricky, if its not active it can be any moment, there is
> nothing serializing against that.
One solution is to keep a filter generation counter and check that on
pmu::add(), and if mis-matched recompute the hardware setup at that
point.
> > + }
> > +
> > + /* matches smp_wmb() in event_sched_in() */
> > + smp_rmb();
> > +
> > + ret = cpu_function_call(READ_ONCE(event->oncpu),
> > + __perf_event_itrace_filters_setup,
> > event);
>
> This otoh, running with IRQs disabled on the CPU the thing is active on
> guarantees it will not become inactive -- nothing can come in and switch
> it off.
>
> > + } while (ret == -EAGAIN);
> > +
> > + return ret;
> > +}
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