perf: Fix event->ctx locking

There have been a few reported issues wrt. the lack of locking around
changing event->ctx. This patch tries to address those.

It avoids the whole rwsem thing; and while it appears to work, please
give it some thought in review.

What I did fail at is sensible runtime checks on the use of
event->ctx, the RCU use makes it very hard.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/20150123125834.209535886@infradead.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
 static int perf_event_period(struct perf_event *event, u64 __user *arg)
{
	struct perf_event_context *ctx = event->ctx;
	int ret = 0, active;
	u64 value;

	if (!is_sampling_event(event))
		return -EINVAL;

	if (copy_from_user(&value, arg, sizeof(value)))
		return -EFAULT;

	if (!value)
		return -EINVAL;

	raw_spin_lock_irq(&ctx->lock);
	if (event->attr.freq) {
		if (value > sysctl_perf_event_sample_rate) {
			ret = -EINVAL;
			goto unlock;
		}

		event->attr.sample_freq = value;
	} else {
		event->attr.sample_period = value;
		event->hw.sample_period = value;
	}

	active = (event->state == PERF_EVENT_STATE_ACTIVE);
	if (active) {
		perf_pmu_disable(ctx->pmu);
		event->pmu->stop(event, PERF_EF_UPDATE);
	}

	local64_set(&event->hw.period_left, 0);

	if (active) {
		event->pmu->start(event, PERF_EF_RELOAD);
		perf_pmu_enable(ctx->pmu);
	}

unlock:
	raw_spin_unlock_irq(&ctx->lock);

	return ret;
}

 static int perf_event_period(struct perf_event *event, u64 __user *arg)
{
	struct perf_event_context *ctx = event->ctx;
	int ret = 0, active;
	u64 value;

	if (!is_sampling_event(event))
		return -EINVAL;

	if (copy_from_user(&value, arg, sizeof(value)))
		return -EFAULT;

	if (!value)
		return -EINVAL;

	raw_spin_lock_irq(&ctx->lock);
	if (event->attr.freq) {
		if (value > sysctl_perf_event_sample_rate) {
			ret = -EINVAL;
			goto unlock;
		}

		event->attr.sample_freq = value;
	} else {
		event->attr.sample_period = value;
		event->hw.sample_period = value;
	}

	active = (event->state == PERF_EVENT_STATE_ACTIVE);
	if (active) {
		perf_pmu_disable(ctx->pmu);
		event->pmu->stop(event, PERF_EF_UPDATE);
	}

	local64_set(&event->hw.period_left, 0);

	if (active) {
		event->pmu->start(event, PERF_EF_RELOAD);
		perf_pmu_enable(ctx->pmu);
	}

unlock:
	raw_spin_unlock_irq(&ctx->lock);

	return ret;
}
