perf, powerpc: Handle events that raise an exception without overflowing

Events on POWER7 can roll back if a speculative event doesn't
eventually complete. Unfortunately in some rare cases they will
raise a performance monitor exception. We need to catch this to
ensure we reset the PMC. In all cases the PMC will be 256 or less
cycles from overflow.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: <stable@kernel.org> # as far back as it applies cleanly
LKML-Reference: <20110309143842.6c22845e@kryten>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
static void hw_perf_event_destroy(struct perf_event *event)
{
	if (!atomic_add_unless(&num_events, -1, 1)) {
		mutex_lock(&pmc_reserve_mutex);
		if (atomic_dec_return(&num_events) == 0)
			release_pmc_hardware();
		mutex_unlock(&pmc_reserve_mutex);
	}
}

static void hw_perf_event_destroy(struct perf_event *event)
{
	if (!atomic_add_unless(&num_events, -1, 1)) {
		mutex_lock(&pmc_reserve_mutex);
		if (atomic_dec_return(&num_events) == 0)
			release_pmc_hardware();
		mutex_unlock(&pmc_reserve_mutex);
	}
}
