void perf_event_update_userpage(struct perf_event *event)
{
	struct perf_event_mmap_page *userpg;
	struct ring_buffer *rb;
	u64 enabled, running, now;

	rcu_read_lock();
	rb = rcu_dereference(event->rb);
	if (!rb)
		goto unlock;

	/*
	 * compute total_time_enabled, total_time_running
	 * based on snapshot values taken when the event
	 * was last scheduled in.
	 *
	 * we cannot simply called update_context_time()
	 * because of locking issue as we can be called in
	 * NMI context
	 */
	calc_timer_values(event, &now, &enabled, &running);

	userpg = rb->user_page;
	/*
	 * Disable preemption so as to not let the corresponding user-space
	 * spin too long if we get preempted.
	 */
	preempt_disable();
	++userpg->lock;
	barrier();
	userpg->index = perf_event_index(event);
	userpg->offset = perf_event_count(event);
	if (userpg->index)
		userpg->offset -= local64_read(&event->hw.prev_count);

	userpg->time_enabled = enabled +
			atomic64_read(&event->child_total_time_enabled);

	userpg->time_running = running +
			atomic64_read(&event->child_total_time_running);

	arch_perf_update_userpage(userpg, now);

	barrier();
	++userpg->lock;
	preempt_enable();
unlock:
	rcu_read_unlock();
}
void perf_event_update_userpage(struct perf_event *event)
{
	struct perf_event_mmap_page *userpg;
	struct ring_buffer *rb;
	u64 enabled, running, now;

	rcu_read_lock();
	rb = rcu_dereference(event->rb);
	if (!rb)
		goto unlock;

	 
	calc_timer_values(event, &now, &enabled, &running);

	userpg = rb->user_page;
	 
	preempt_disable();
	++userpg->lock;
	barrier();
	userpg->index = perf_event_index(event);
	userpg->offset = perf_event_count(event);
	if (userpg->index)
		userpg->offset -= local64_read(&event->hw.prev_count);

	userpg->time_enabled = enabled +
			atomic64_read(&event->child_total_time_enabled);

	userpg->time_running = running +
			atomic64_read(&event->child_total_time_running);

	arch_perf_update_userpage(userpg, now);

	barrier();
	++userpg->lock;
	preempt_enable();
unlock:
	rcu_read_unlock();
}