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.
> + }
> +
> + /* 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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