ftrace: preemptoff selftest not working

Impact: fix preemptoff and preemptirqsoff tracer self-tests

I was wondering why the preemptoff and preemptirqsoff tracer selftests
don't work on s390. After all its just that they get called from
non-preemptible context:

kernel_init() will execute all initcalls, however the first line in
kernel_init() is lock_kernel(), which causes the preempt_count to be
increased. Any later calls to add_preempt_count() (especially those
from the selftests) will therefore not result in a call to
trace_preempt_off() since the check below in add_preempt_count()
will be false:

        if (preempt_count() == val)
                trace_preempt_off(CALLER_ADDR0, get_parent_ip(CALLER_ADDR1));

Hence the trace buffer will be empty.

Fix this by releasing the BKL during the self-tests.
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace.c

@@ -532,6 +532,13 @@ int register_tracer(struct tracer *type)
 	}
 
 #ifdef CONFIG_FTRACE_STARTUP_TEST
+	/*
+	 * When this gets called we hold the BKL which means that preemption
+	 * is disabled. Various trace selftests however need to disable
+	 * and enable preemption for successful tests. So we drop the BKL here
+	 * and grab it after the tests again.
+	 */
+	unlock_kernel();
 	if (type->selftest) {
 		struct tracer *saved_tracer = current_trace;
 		struct trace_array *tr = &global_trace;
@@ -562,6 +569,7 @@ int register_tracer(struct tracer *type)
 		}
 		printk(KERN_CONT "PASSED\n");
 	}
+	lock_kernel();
 #endif
 
 	type->next = trace_types;