============================================
WARNING: possible recursive locking detected
5.14.0-rc4-syzkaller #0 Not tainted
--------------------------------------------
systemd-udevd/18631 is trying to acquire lock:
ffff888013d35118 (&disk->open_mutex){+.+.}-{3:3}, at: del_gendisk+0x8b/0x770 block/genhd.c:587

but task is already holding lock:
ffff888013d35118 (&disk->open_mutex){+.+.}-{3:3}, at: blkdev_put+0xa0/0xae0 fs/block_dev.c:1534

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&disk->open_mutex);
  lock(&disk->open_mutex);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

2 locks held by systemd-udevd/18631:
 #0: ffff888013d35118 (&disk->open_mutex){+.+.}-{3:3}, at: blkdev_put+0xa0/0xae0 fs/block_dev.c:1534
 #1: ffffffff8c4896a8 (nbd_index_mutex){+.+.}-{3:3}, at: refcount_dec_and_mutex_lock lib/refcount.c:118 [inline]
 #1: ffffffff8c4896a8 (nbd_index_mutex){+.+.}-{3:3}, at: refcount_dec_and_mutex_lock+0x50/0x140 lib/refcount.c:113

stack backtrace:
CPU: 3 PID: 18631 Comm: systemd-udevd Not tainted 5.14.0-rc4-syzkaller #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
Call Trace:
 __dump_stack lib/dump_stack.c:88 [inline]
 dump_stack_lvl+0xcd/0x134 lib/dump_stack.c:105
 print_deadlock_bug kernel/locking/lockdep.c:2944 [inline]
 check_deadlock kernel/locking/lockdep.c:2987 [inline]
 validate_chain kernel/locking/lockdep.c:3776 [inline]
 __lock_acquire.cold+0x149/0x3ab kernel/locking/lockdep.c:5015
 lock_acquire kernel/locking/lockdep.c:5625 [inline]
 lock_acquire+0x1ab/0x510 kernel/locking/lockdep.c:5590
 __mutex_lock_common kernel/locking/mutex.c:959 [inline]
 __mutex_lock+0x12a/0x10a0 kernel/locking/mutex.c:1104
 del_gendisk+0x8b/0x770 block/genhd.c:587
 nbd_dev_remove drivers/block/nbd.c:241 [inline]
 nbd_put.part.0+0x82/0x160 drivers/block/nbd.c:263
 nbd_put drivers/block/nbd.c:1527 [inline]
 nbd_release+0x14d/0x1a0 drivers/block/nbd.c:1526
 blkdev_put_whole fs/block_dev.c:1279 [inline]
 blkdev_put+0x34f/0xae0 fs/block_dev.c:1576
 blkdev_close+0x8c/0xb0 fs/block_dev.c:1586
 __fput+0x288/0x920 fs/file_table.c:280
 task_work_run+0xdd/0x1a0 kernel/task_work.c:164
 tracehook_notify_resume include/linux/tracehook.h:189 [inline]
 exit_to_user_mode_loop kernel/entry/common.c:175 [inline]
 exit_to_user_mode_prepare+0x27e/0x290 kernel/entry/common.c:209
 __syscall_exit_to_user_mode_work kernel/entry/common.c:291 [inline]
 syscall_exit_to_user_mode+0x19/0x60 kernel/entry/common.c:302
 do_syscall_64+0x42/0xb0 arch/x86/entry/common.c:86
 entry_SYSCALL_64_after_hwframe+0x44/0xae
RIP: 0033:0x7f5e2b96c270
Code: 73 01 c3 48 8b 0d 38 7d 20 00 f7 d8 64 89 01 48 83 c8 ff c3 66 0f 1f 44 00 00 83 3d 59 c1 20 00 00 75 10 b8 03 00 00 00 0f 05 <48> 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 ee fb ff ff 48 89 04 24
RSP: 002b:00007ffd980d07f8 EFLAGS: 00000246 ORIG_RAX: 0000000000000003
RAX: 0000000000000000 RBX: 0000000000000007 RCX: 00007f5e2b96c270
RDX: 000000000aba9500 RSI: 0000000000000000 RDI: 0000000000000007
RBP: 00007f5e2c826710 R08: 000000000000004a R09: 0000000000000008
R10: 00005620a9a377f8 R11: 0000000000000246 R12: 0000000000000000
R13: 00005620a9a3a920 R14: 0000000000000003 R15: 000000000000000e