ocfs2/dlm: fix possible convertion deadlock

We found there is a convertion deadlock when the owner of lockres happened to crash before send DLM_PROXY_AST_MSG for a downconverting lock. The situation is as follows:

Node1 Node2 Node3
the owner of lockresA
lock_1 granted at EX mode
and call ocfs2_cluster_unlock
to decrease ex_holders.
converting lock_3 from NL to EX
send DLM_PROXY_AST_MSG
to Node1, asking Node 1
to downconvert.
receiving DLM_PROXY_AST_MSG,
thread ocfs2dc send
DLM_CONVERT_LOCK_MSG
to Node2 to downconvert
lock_1(EX->NL).
lock_1 can be granted and
put it into pending_asts
list, return DLM_NORMAL.
then something happened
and Node2 crashed.
received DLM_NORMAL, waiting
for DLM_PROXY_AST_MSG.
selected as the recovery master, receving migrate lock from Node1, queue lock_1 to the tail of converting list.

After dlm recovery, converting list in the master of lockresA(Node3) will be: converting list head <-> lock_3(NL->EX) <->lock_1(EX<->NL). Requested mode of lock_3 is not compatible with the granted mode of lock_1, so it can not be granted. and lock_1 can not downconvert because covnerting queue is strictly FIFO. So a deadlock is created. We think function dlm_process_recovery_data() should queue_ast for lock_1 or alter the order of lock_1 and lock_3, so dlm_thread can process lock_1 first. And if there are multiple downconverting locks, they must convert form PR to NL, so no need to sort them.

---
 fs/ocfs2/dlm/dlmrecovery.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

index fe29f79..5de0194 100644
@@ -1986,7 +1986,15 @@ skip_lvb:
 		}
 		if (!bad) {
 			dlm_lock_get(newlock);
-			list_add_tail(&newlock->list, queue);
+			if (mres->flags & DLM_MRES_RECOVERY &&
+					ml->list == DLM_CONVERTING_LIST &&
+					newlock->ml.type >
+					newlock->ml.convert_type) {
+				/* newlock is doing downconvert, add it to the
+				 * head of converting list */
+				list_add(&newlock->list, queue);
+			} else
+				list_add_tail(&newlock->list, queue);
 			mlog(0, "%s:%.*s: added lock for node %u, "
 			     "setting refmap bit\n", dlm->name,
 			     res->lockname.len, res->lockname.name, ml->node);

1.8.3.4