Cifs files may be put into fileinfo_put_wq during umounting cifs.
After umount done, cifsFileInfo_put_final is called, which cause
following BUG:

BUG: kernel NULL pointer dereference, address: 0000000000000000
...
[  134.222152]  list_lru_add+0x64/0x1a0
[  134.222399]  ? cifs_put_tcon+0x171/0x340 [cifs]
[  134.222772]  d_lru_add+0x44/0x60
[  134.222997]  dput+0x1fc/0x210
[  134.223213]  cifsFileInfo_put_final+0x11a/0x140 [cifs]
[  134.223576]  process_one_work+0x17c/0x320
[  134.223843]  worker_thread+0x188/0x280
[  134.224084]  ? __pfx_worker_thread+0x10/0x10
[  134.224366]  kthread+0xcc/0x100
[  134.224576]  ? __pfx_kthread+0x10/0x10
[  134.224827]  ret_from_fork+0x30/0x50
[  134.225063]  ? __pfx_kthread+0x10/0x10
[  134.225328]  ret_from_fork_asm+0x1b/0x30

This can be reproduce by following:
unshare -n bash -c "
mkdir -p ${CIFS_MNT}
ip netns attach root 1
ip link add eth0 type veth peer veth0 netns root
ip link set eth0 up
ip -n root link set veth0 up
ip addr add 192.168.0.2/24 dev eth0
ip -n root addr add 192.168.0.1/24 dev veth0
ip route add default via 192.168.0.1 dev eth0
ip netns exec root sysctl net.ipv4.ip_forward=1
ip netns exec root iptables -t nat -A POSTROUTING -s 192.168.0.2 -o
${DEV} -j MASQUERADE
mount -t cifs ${CIFS_PATH} ${CIFS_MNT} -o
vers=3.0,sec=ntlmssp,credentials=${CIFS_CRED},rsize=65536,wsize=65536,cache=none,echo_interval=1
touch ${CIFS_MNT}/a.txt
ip netns exec root iptables -t nat -D POSTROUTING -s 192.168.0.2 -o
${DEV} -j MASQUERADE
"
umount ${CIFS_MNT}

Fixes: 340cea84f69 ("cifs: open files should not hold ref on superblock")

Signed-off-by: Jian Zhang <zhangjian496@huawei.com>
---
 fs/smb/client/connect.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/smb/client/connect.c b/fs/smb/client/connect.c
index dcde25d..7d09f07 100644
--- a/fs/smb/client/connect.c
+++ b/fs/smb/client/connect.c
@@ -3996,6 +3996,9 @@ cifs_umount(struct cifs_sb_info *cifs_sb)
 	}
 	spin_unlock(&cifs_sb->tlink_tree_lock);

+    flush_workqueue(serverclose_wq);
+    flush_workqueue(fileinfo_put_wq);
+
 	kfree(cifs_sb->prepath);
 	call_rcu(&cifs_sb->rcu, delayed_free);
 }
-- 
2.23.0