Re: [PATCH v0 3/5] perf: Introduce instruction trace filtering

[Peter Zijlstra <peterz@infradead.org>]

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;
> +}