perf_events: Fix broken event grouping

Events were not grouped anymore. The reason was that in
perf_event_open(), the field event->group_leader was
initialized before the function looked up the group_fd
to find the event leader. This patch fixes this by
reordering the code correctly.

Signed-off-by: Stephane Eranian <eranian@google.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Robert Richter <robert.richter@amd.com>
LKML-Reference: <20100917093009.360420946@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/perf_event.c

@@ -5550,17 +5550,11 @@ SYSCALL_DEFINE5(perf_event_open,
 	if (event_fd < 0)
 		return event_fd;
 
-	event = perf_event_alloc(&attr, cpu, group_leader, NULL, NULL);
-	if (IS_ERR(event)) {
-		err = PTR_ERR(event);
-		goto err_fd;
-	}
-
 	if (group_fd != -1) {
 		group_leader = perf_fget_light(group_fd, &fput_needed);
 		if (IS_ERR(group_leader)) {
 			err = PTR_ERR(group_leader);
-			goto err_alloc;
+			goto err_fd;
 		}
 		group_file = group_leader->filp;
 		if (flags & PERF_FLAG_FD_OUTPUT)
@@ -5569,6 +5563,12 @@ SYSCALL_DEFINE5(perf_event_open,
 			group_leader = NULL;
 	}
 
+	event = perf_event_alloc(&attr, cpu, group_leader, NULL, NULL);
+	if (IS_ERR(event)) {
+		err = PTR_ERR(event);
+		goto err_fd;
+	}
+
 	/*
 	 * Special case software events and allow them to be part of
 	 * any hardware group.
@@ -5653,7 +5653,6 @@ err_context:
 	put_ctx(ctx);
 err_group_fd:
 	fput_light(group_file, fput_needed);
-err_alloc:
 	free_event(event);
 err_fd:
 	put_unused_fd(event_fd);