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SubjectWARNING in kernfs_remove_by_name_ns

syzbot found the following crash on:

HEAD commit: abf02e29 Merge tag 'pm-5.2-rc6' of git://
git tree: upstream
console output:
kernel config:
dashboard link:
compiler: gcc (GCC) 9.0.0 20181231 (experimental)

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

IMPORTANT: if you fix the bug, please add the following tag to the commit:

------------[ cut here ]------------
kernfs: can not remove 'nr_tags', no directory
WARNING: CPU: 0 PID: 19627 at fs/kernfs/dir.c:1503
kernfs_remove_by_name_ns+0x9f/0xb0 fs/kernfs/dir.c:1503
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 19627 Comm: syz-executor.1 Not tainted 5.2.0-rc5+ #31
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+0x172/0x1f0 lib/dump_stack.c:113
panic+0x2cb/0x744 kernel/panic.c:219
__warn.cold+0x20/0x4d kernel/panic.c:576
report_bug+0x263/0x2b0 lib/bug.c:186
fixup_bug arch/x86/kernel/traps.c:179 [inline]
fixup_bug arch/x86/kernel/traps.c:174 [inline]
do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:272
do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:291
invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:986
RIP: 0010:kernfs_remove_by_name_ns+0x9f/0xb0 fs/kernfs/dir.c:1503
Code: 99 ff 48 c7 c7 60 22 c9 88 41 bc fe ff ff ff e8 a7 c4 44 05 eb d7 e8
f0 4e 99 ff 4c 89 ee 48 c7 c7 80 c8 78 87 e8 ff 87 6b ff <0f> 0b 41 bc fe
ff ff ff eb b9 0f 1f 80 00 00 00 00 55 48 89 e5 41
RSP: 0018:ffff888058aaf798 EFLAGS: 00010286
RAX: 0000000000000000 RBX: ffffffff88f18128 RCX: 0000000000000000
RDX: 000000000001e0a4 RSI: ffffffff815ad926 RDI: ffffed100b155ee5
RBP: ffff888058aaf7b0 R08: ffff88804eb8a240 R09: ffffed1015d060a1
R10: ffffed1015d060a0 R11: ffff8880ae830507 R12: 0000000000000000
R13: ffffffff87a35540 R14: 0000000000000000 R15: ffffffff87a35480
kernfs_remove_by_name include/linux/kernfs.h:567 [inline]
remove_files.isra.0+0x7b/0x190 fs/sysfs/group.c:27
sysfs_remove_group+0xb6/0x1b0 fs/sysfs/group.c:264
sysfs_remove_groups fs/sysfs/group.c:288 [inline]
sysfs_remove_groups+0x61/0xb0 fs/sysfs/group.c:280
kobject_del+0x9c/0x170 lib/kobject.c:619
blk_mq_unregister_hctx.part.0+0x126/0x170 block/blk-mq-sysfs.c:243
blk_mq_unregister_hctx block/blk-mq-sysfs.c:237 [inline]
blk_mq_sysfs_unregister+0x16b/0x200 block/blk-mq-sysfs.c:373
__blk_mq_update_nr_hw_queues block/blk-mq.c:3286 [inline]
blk_mq_update_nr_hw_queues+0x4f9/0xd30 block/blk-mq.c:3321
nbd_start_device+0x156/0xac0 drivers/block/nbd.c:1166
nbd_start_device_ioctl drivers/block/nbd.c:1207 [inline]
__nbd_ioctl drivers/block/nbd.c:1278 [inline]
nbd_ioctl+0x53f/0xbd0 drivers/block/nbd.c:1318
__blkdev_driver_ioctl block/ioctl.c:304 [inline]
blkdev_ioctl+0xece/0x1c10 block/ioctl.c:606
block_ioctl+0xee/0x130 fs/block_dev.c:1930
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:509 [inline]
do_vfs_ioctl+0xd5f/0x1380 fs/ioctl.c:696
ksys_ioctl+0xab/0xd0 fs/ioctl.c:713
__do_sys_ioctl fs/ioctl.c:720 [inline]
__se_sys_ioctl fs/ioctl.c:718 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:718
do_syscall_64+0xfd/0x680 arch/x86/entry/common.c:301
RIP: 0033:0x4592c9
Code: fd b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 cb b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f9aba72cc78 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00000000004592c9
RDX: 0000000000000000 RSI: 000000000000ab03 RDI: 0000000000000004
RBP: 000000000075bfc8 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f9aba72d6d4
R13: 00000000004c2a81 R14: 00000000004d5b10 R15: 00000000ffffffff
Kernel Offset: disabled
Rebooting in 86400 seconds..

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