Ocfs2: Handle empty list in lockres_seq_start() for dlmdebug.c

This patch tries to handle the case in which list 'dlm->tracking_list' is
empty, to avoid accessing an invalid pointer. It fixes the following oops:

http://oss.oracle.com/bugzilla/show_bug.cgi?id=1287

Signed-off-by: Tristan Ye <tristan.ye@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/fs/ocfs2/dlm/dlmdebug.c b/fs/ocfs2/dlm/dlmdebug.c
index 5efdd37dfe48..901ca52bf86b 100644
--- a/fs/ocfs2/dlm/dlmdebug.c
+++ b/fs/ocfs2/dlm/dlmdebug.c
@@ -636,8 +636,14 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
         spin_lock(&dlm->track_lock);
         if (oldres)
                 track_list = &oldres->tracking;
-        else
+        else {
                 track_list = &dlm->tracking_list;
+                if (list_empty(track_list)) {
+                        dl = NULL;
+                        spin_unlock(&dlm->track_lock);
+                        goto bail;
+                }
+        }
 
         list_for_each_entry(res, track_list, tracking) {
                 if (&res->tracking == &dlm->tracking_list)
@@ -660,6 +666,7 @@ static void *lockres_seq_start(struct seq_file *m, loff_t *pos)
         } else
                 dl = NULL;
 
+bail:
         /* passed to seq_show */
         return dl;
 }