BUG: Dentry ADDR{i=NUM,n=/} still in use (-NUM) [unmount of erofs loop4]

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 59456c9cc40c Linux 4.19.204
git tree: linux-4.19.y
console output: https://syzkaller.appspot.com/x/log.txt?x=173c42c1300000
kernel config: https://syzkaller.appspot.com/x/.config?x=a84b4514346e13dc
dashboard link: https://syzkaller.appspot.com/bug?extid=921f7144ac4cbe0b184f
compiler: gcc version 10.2.1 20210110 (Debian 10.2.1-6)

Unfortunately, I don't have any reproducer for this issue yet.

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+921f71...@syzkaller.appspotmail.com

erofs: root inode @ nid 36
erofs: unmounted for /dev/loop2
erofs: mounted on /dev/loop5 with opts: .
erofs: unmounted for /dev/loop5
block nbd0: shutting down sockets
BUG: Dentry 00000000009714f5{i=0,n=/} still in use (-128) [unmount of erofs loop4]
hpfs: bad mount options.
------------[ cut here ]------------
WARNING: CPU: 0 PID: 23800 at fs/dcache.c:1518 umount_check fs/dcache.c:1518 [inline]
WARNING: CPU: 0 PID: 23800 at fs/dcache.c:1518 umount_check.cold+0xf2/0x116 fs/dcache.c:1499
Kernel panic - not syncing: panic_on_warn set ...

CPU: 0 PID: 23800 Comm: syz-executor.4 Not tainted 4.19.204-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x1fc/0x2ef lib/dump_stack.c:118
panic+0x26a/0x50e kernel/panic.c:186
__warn.cold+0x20/0x5a kernel/panic.c:541
report_bug+0x262/0x2b0 lib/bug.c:183
fixup_bug arch/x86/kernel/traps.c:178 [inline]
fixup_bug arch/x86/kernel/traps.c:173 [inline]
do_error_trap+0x1d7/0x310 arch/x86/kernel/traps.c:296
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:1038
RIP: 0010:umount_check fs/dcache.c:1518 [inline]
RIP: 0010:umount_check.cold+0xf2/0x116 fs/dcache.c:1499
Code: f9 4d 89 f1 45 89 e0 48 89 e9 41 55 4c 89 fa 48 89 ee 48 c7 c7 80 a8 74 88 e8 99 f0 fe ff 48 c7 c7 80 a7 74 88 e8 8d f0 fe ff <0f> 0b 58 e9 3e 29 b4 f9 e8 7a d5 a3 f9 e9 22 ff ff ff 48 89 df e8
RSP: 0018:ffff8880462a7a98 EFLAGS: 00010286
RAX: 0000000000000024 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000040000 RSI: ffffffff814e0d61 RDI: ffffed1008c54f45
RBP: ffff8880a1d53600 R08: 0000000000000024 R09: 0000000000000000
R10: 0000000000000005 R11: 0000000000000000 R12: 00000000ffffff80
R13: ffff88809b181350 R14: ffffffff893f9e40 R15: 0000000000000000
d_walk+0x196/0x990 fs/dcache.c:1253
do_one_tree fs/dcache.c:1525 [inline]
shrink_dcache_for_umount+0x87/0x330 fs/dcache.c:1541
generic_shutdown_super+0x68/0x370 fs/super.c:441
kill_block_super+0x97/0xf0 fs/super.c:1185
deactivate_locked_super+0x94/0x160 fs/super.c:329
mount_bdev+0x373/0x3b0 fs/super.c:1160
erofs_mount+0x8c/0xc0 drivers/staging/erofs/super.c:512
mount_fs+0xa3/0x310 fs/super.c:1261
vfs_kern_mount.part.0+0x68/0x470 fs/namespace.c:961
vfs_kern_mount fs/namespace.c:951 [inline]
do_new_mount fs/namespace.c:2483 [inline]
do_mount+0x113c/0x2f10 fs/namespace.c:2813
ksys_mount+0xcf/0x130 fs/namespace.c:3029
__do_sys_mount fs/namespace.c:3043 [inline]
__se_sys_mount fs/namespace.c:3040 [inline]
__x64_sys_mount+0xba/0x150 fs/namespace.c:3040
do_syscall_64+0xf9/0x620 arch/x86/entry/common.c:293
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x467b0a
Code: 48 c7 c2 bc ff ff ff f7 d8 64 89 02 b8 ff ff ff ff eb d2 e8 b8 04 00 00 0f 1f 84 00 00 00 00 00 49 89 ca b8 a5 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 01 c3 48 c7 c1 bc ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007fee31babfa8 EFLAGS: 00000202 ORIG_RAX: 00000000000000a5
RAX: ffffffffffffffda RBX: 0000000020000200 RCX: 0000000000467b0a
RDX: 0000000020000000 RSI: 0000000020000100 RDI: 00007fee31bac000
RBP: 00007fee31bac040 R08: 00007fee31bac040 R09: 0000000020000000
R10: 0000000000000000 R11: 0000000000000202 R12: 0000000020000000
R13: 0000000020000100 R14: 00007fee31bac000 R15: 0000000020010a00
Kernel Offset: disabled
Rebooting in 86400 seconds..


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.