tracing: Comment why cond_snapshot is checked outside of max_lock protection

Before setting tr->cond_snapshot, it must be NULL before it can be updated.
It can go to NULL when a trace event hist trigger is created or removed, and
can only be modified under the max_lock spin lock. But because it can only
be set to something other than NULL under both the max_lock spin lock as
well as the trace_types_lock, we can perform the check if it is not NULL
only under the trace_types_lock and fail out without having to grab the
max_lock spin lock.

This is very subtle, and deserves a comment.
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -1116,6 +1116,14 @@ int tracing_snapshot_cond_enable(struct trace_array *tr, void *cond_data,
 		goto fail_unlock;
 	}
 
+	/*
+	 * The cond_snapshot can only change to NULL without the
+	 * trace_types_lock. We don't care if we race with it going
+	 * to NULL, but we want to make sure that it's not set to
+	 * something other than NULL when we get here, which we can
+	 * do safely with only holding the trace_types_lock and not
+	 * having to take the max_lock.
+	 */
 	if (tr->cond_snapshot) {
 		ret = -EBUSY;
 		goto fail_unlock;