tracing/kprobes: unregister_trace_probe needs to be called under mutex

Comment in unregister_trace_probe() says probe_lock will be held when it
gets called. However there is a case where it might called without the
probe_lock being held. Also since we are traversing the probe_list and
deleting an element from the probe_list, probe_lock should be held.

This was first pointed in uprobes traceevent review by Frederic
Weisbecker here.  (http://lkml.org/lkml/2010/5/12/106)

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
LKML-Reference: <20100630084548.GA10325@linux.vnet.ibm.com>
Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

kernel/trace/trace_kprobe.c

@@ -925,14 +925,17 @@ static int create_trace_probe(int argc, char **argv)
 			pr_info("Delete command needs an event name.\n");
 			return -EINVAL;
 		}
+		mutex_lock(&probe_lock);
 		tp = find_probe_event(event, group);
 		if (!tp) {
+			mutex_unlock(&probe_lock);
 			pr_info("Event %s/%s doesn't exist.\n", group, event);
 			return -ENOENT;
 		}
 		/* delete an event */
 		unregister_trace_probe(tp);
 		free_trace_probe(tp);
+		mutex_unlock(&probe_lock);
 		return 0;
 	}