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:

> @@ -559,6 +590,10 @@ struct perf_event {
>  
>  	atomic_t			event_limit;
>  
> +	/* instruction trace filters */
> +	struct list_head		itrace_filters;
> +	struct mutex			itrace_filters_mutex;
> +
>  	void (*destroy)(struct perf_event *);
>  	struct rcu_head			rcu_head;
>  

> +static int __perf_event_itrace_filters_setup(void *info)
> +{
> +	struct perf_event *event = info;
> +	int ret;
> +
> +	if (READ_ONCE(event->state) != PERF_EVENT_STATE_ACTIVE)
> +		return -EAGAIN;
> +
> +	/* matches smp_wmb() in event_sched_in() */
> +	smp_rmb();
> +
> +	/*
> +	 * There is a window with interrupts enabled before we get here,
> +	 * so we need to check again lest we try to stop another cpu's event.
> +	 */
> +	if (READ_ONCE(event->oncpu) != smp_processor_id())
> +		return -EAGAIN;
> +
> +	event->pmu->stop(event, PERF_EF_UPDATE);
> +	rcu_read_lock();

So you're holding rcu_read_lock() here to ensure the filter list is
observable. However this is still very much racy, nothing stops another
filter being added while we're trying to validate/program the hardware.

The solution we've used for other such places in perf is to use both a
mutex and a spinlock to protect the list. You need to hold both to
modify a list, holding either ensures the list is stable.

That would allow you to hold the spinlock here, and call the pmu method
on a stable list.

> +	ret = event->pmu->itrace_filter_setup(event);
> +	rcu_read_unlock();
> +	event->pmu->start(event, PERF_EF_RELOAD);
> +
> +	return ret;
> +}

> +/*
> + * Insert an itrace @filter into @event's list of filters.
> + * @filter is used as a template
> + */
> +static int perf_itrace_filter_insert(struct perf_event *event,
> +				     struct perf_itrace_filter *src,
> +				     struct task_struct *task)
> +{

> +	/*
> +	 * If we're called through perf_itrace_filters_clone(), we're already
> +	 * holding parent's filter mutex.
> +	 */
> +	mutex_lock_nested(&event->itrace_filters_mutex, SINGLE_DEPTH_NESTING);
> +	list_add_tail_rcu(&filter->entry, &event->itrace_filters);
> +	mutex_unlock(&event->itrace_filters_mutex);
> +
> +	return 0;
> +}