[PATCH -rt] fix preempt count underflow in user_trace_stop

[Michal Schmidt <mschmidt@redhat.com>]

When playing with trace_user_trigger_irq in order to trace
IRQ->userspace latencies, I encountered a bug in the latency tracer. If
I have wakeup_timing enabled and attempt to stop the trace in my
userspace program, the system crashes. This is caused by an unbalanced
preempt_enable() which underflows the preempt count.

Signed-off-by: Michal Schmidt <mschmidt@redhat.com>

diff --git a/kernel/latency_trace.c b/kernel/latency_trace.c
index e07bb95..29dfb79 100644
--- a/kernel/latency_trace.c
+++ b/kernel/latency_trace.c
@@ -2396,7 +2396,6 @@ long user_trace_stop(void)
 		if (current != sch.task) {
 			__raw_spin_unlock(&sch.trace_lock);
 			local_irq_restore(flags);
-			preempt_enable();
 			return -EINVAL;
 		}
 		sch.task = NULL;