ocfs2: dlm: fix race between purge and get lock resource

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 <[email protected]>

Cc Joseph Qi <[email protected]>

Cc <[email protected]>

---
 fs/ocfs2/dlm/dlmmaster.c |   13 +++++++++++++
 1 file changed, 13 insertions(+)

fs/ocfs2/dlm/dlmmaster.c +13 -0

index a6944b2..fdf4b41 100644
@@ -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);