perf namespace: Protect reading thread's namespace

It seems that the current code lacks holding the namespace lock in
thread__namespaces().  Otherwise it can see inconsistent results.
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Hari Bathini <hbathini@linux.vnet.ibm.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Link: http://lkml.kernel.org/r/20190522053250.207156-2-namhyung@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/thread.c

@@ -133,7 +133,7 @@ void thread__put(struct thread *thread)
 	}
 }
 
-struct namespaces *thread__namespaces(const struct thread *thread)
+static struct namespaces *__thread__namespaces(const struct thread *thread)
 {
 	if (list_empty(&thread->namespaces_list))
 		return NULL;
@@ -141,10 +141,21 @@ struct namespaces *thread__namespaces(const struct thread *thread)
 	return list_first_entry(&thread->namespaces_list, struct namespaces, list);
 }
 
+struct namespaces *thread__namespaces(const struct thread *thread)
+{
+	struct namespaces *ns;
+
+	down_read((struct rw_semaphore *)&thread->namespaces_lock);
+	ns = __thread__namespaces(thread);
+	up_read((struct rw_semaphore *)&thread->namespaces_lock);
+
+	return ns;
+}
+
 static int __thread__set_namespaces(struct thread *thread, u64 timestamp,
 				    struct namespaces_event *event)
 {
-	struct namespaces *new, *curr = thread__namespaces(thread);
+	struct namespaces *new, *curr = __thread__namespaces(thread);
 
 	new = namespaces__new(event);
 	if (!new)