[PATCH dlm] dlm: fix add msg handle in send_queue ordered

[Alexander Aring <aahringo@redhat.com>]

In a benchmark scenario triggering a lot of requests that triggers a lot
of DLM messages on the network it can be that the mh->seq is not ordered
according the oldest seq number. This ordering is required by
dlm_receive_ack as "before(mh->seq, seq)" will stop to check for older
sequence numbers that are ordered in the tail of "node->send_queue".

The side effects of not having it correct ordered regarding
"before(mh->seq, seq)" are refcounting issues and use-after free.

I only was able to reproduce this issue in a experimental DLM branch
and a user space DLM benchmark that uses io_uring. After changing this I
don't experienced any refcounting with the sending buffer issues anymore.

Fixes: 489d8e559c659 ("fs: dlm: add reliable connection if reconnect")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
---
 fs/dlm/midcomms.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/dlm/midcomms.c b/fs/dlm/midcomms.c
index a5b363b4785f3..6b522d4e2b7e3 100644
--- a/fs/dlm/midcomms.c
+++ b/fs/dlm/midcomms.c
@@ -969,10 +969,10 @@ static void midcomms_new_msg_cb(void *data)
 	atomic_inc(&mh->node->send_queue_cnt);
 
 	spin_lock_bh(&mh->node->send_queue_lock);
+	/* need to be locked with list_add_tail_rcu() because list is ordered */
+	mh->seq = atomic_fetch_inc(&mh->node->seq_send);
 	list_add_tail_rcu(&mh->list, &mh->node->send_queue);
 	spin_unlock_bh(&mh->node->send_queue_lock);
-
-	mh->seq = atomic_fetch_inc(&mh->node->seq_send);
 }
 
 static struct dlm_msg *dlm_midcomms_get_msg_3_2(struct dlm_mhandle *mh, int nodeid,
-- 
2.43.0