[Ocfs2-devel] [PATCH] ocfs2: dlm: fix race between purge and get lock resource

[Junxiao Bi]

There is a race window in dlm_get_lock_resource(), which may
return a lock resource which have been purged. This will cause
the process hung forever in dlmlock() as the ast msg can't be
handled due to its lock resource not exist.

dlm_get_lock_resource {
	...
	spin_lock(&dlm->spinlock);
	tmpres = __dlm_lookup_lockres_full(dlm, lockid, namelen, hash);
	if (tmpres) {
		spin_unlock(&dlm->spinlock);
		>>>>>>>> race window, dlm_run_purge_list() may run and purge
				 the lock resource
		spin_lock(&tmpres->spinlock);
		...
		spin_unlock(&tmpres->spinlock);
	}
}

Signed-off-by: Junxiao Bi <junxiao.bi at oracle.com>
Cc: Joseph Qi <joseph.qi at huawei.com>
Cc: <stable at vger.kernel.org>
---
 fs/ocfs2/dlm/dlmmaster.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/ocfs2/dlm/dlmmaster.c b/fs/ocfs2/dlm/dlmmaster.c
index a6944b2..fdf4b41 100644
--- a/fs/ocfs2/dlm/dlmmaster.c
+++ b/fs/ocfs2/dlm/dlmmaster.c
@@ -757,6 +757,19 @@ lookup:
 	if (tmpres) {
 		spin_unlock(&dlm->spinlock);
 		spin_lock(&tmpres->spinlock);
+
+		/*
+		 * Right after dlm spinlock was released, dlm_thread could have
+		 * purged the lockres. Check if lockres got unhashed. If so
+		 * start over.
+		 */
+		if (hlist_unhashed(&tmpres->hash_node)) {
+			spin_unlock(&tmpres->spinlock);
+			dlm_lockres_put(tmpres);
+			tmpres = NULL;
+			goto lookup;
+		}
+
 		/* Wait on the thread that is mastering the resource */
 		if (tmpres->owner == DLM_LOCK_RES_OWNER_UNKNOWN) {
 			__dlm_wait_on_lockres(tmpres);
-- 
1.7.9.5