[Bugfix] unregister_trace_probe needs to be called under mutex

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

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)

This patch is needed for both 2.6.35-rc3 and 2.6.35-rc3-tip

Signed-off-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
---

diff --git a/kernel/trace/trace_kprobe.c b/kernel/trace/trace_kprobe.c
index 4f11a56..67670cd 100644
--- a/kernel/trace/trace_kprobe.c
+++ b/kernel/trace/trace_kprobe.c
@@ -269,14 +269,17 @@ 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;
 	}