[syzbot] [block?] possible deadlock in __del_gendisk

[syzbot <syzbot+2e9e529ac0b319316453@syzkaller.appspotmail.com>]

Hello,

syzbot found the following issue on:

HEAD commit:    3be1a7a31fbd Add linux-next specific files for 20250526
git tree:       linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=1573bad4580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=9ab703e8a19430df
dashboard link: https://syzkaller.appspot.com/bug?extid=2e9e529ac0b319316453
compiler:       Debian clang version 20.1.6 (++20250514063057+1e4d39e07757-1~exp1~20250514183223.118), Debian LLD 20.1.6

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

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/33e6b012d232/disk-3be1a7a3.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/f92328298470/vmlinux-3be1a7a3.xz
kernel image: https://storage.googleapis.com/syzbot-assets/e5cbdb6f4a5c/bzImage-3be1a7a3.xz

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+2e9e529ac0b319316453@syzkaller.appspotmail.com

======================================================
WARNING: possible circular locking dependency detected
6.15.0-rc7-next-20250526-syzkaller #0 Not tainted
------------------------------------------------------
syz.4.1558/11720 is trying to acquire lock:
ffff888142bb3358 (&disk->open_mutex){+.+.}-{4:4}, at: __del_gendisk+0x129/0x9e0 block/genhd.c:706

but task is already holding lock:
ffff888142bb2368 (&set->update_nr_hwq_lock){++++}-{4:4}, at: del_gendisk+0xe0/0x160 block/genhd.c:818

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #2 (&set->update_nr_hwq_lock){++++}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
       down_write+0x96/0x1f0 kernel/locking/rwsem.c:1577
       blk_mq_update_nr_hw_queues+0x3b/0x14c0 block/blk-mq.c:5041
       nbd_start_device+0x16c/0xac0 drivers/block/nbd.c:1476
       nbd_genl_connect+0x1250/0x1930 drivers/block/nbd.c:2201
       genl_family_rcv_msg_doit+0x215/0x300 net/netlink/genetlink.c:1115
       genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
       genl_rcv_msg+0x60e/0x790 net/netlink/genetlink.c:1210
       netlink_rcv_skb+0x208/0x470 net/netlink/af_netlink.c:2534
       genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
       netlink_unicast_kernel net/netlink/af_netlink.c:1313 [inline]
       netlink_unicast+0x75b/0x8d0 net/netlink/af_netlink.c:1339
       netlink_sendmsg+0x805/0xb30 net/netlink/af_netlink.c:1883
       sock_sendmsg_nosec net/socket.c:712 [inline]
       __sock_sendmsg+0x21c/0x270 net/socket.c:727
       ____sys_sendmsg+0x505/0x830 net/socket.c:2566
       ___sys_sendmsg+0x21f/0x2a0 net/socket.c:2620
       __sys_sendmsg net/socket.c:2652 [inline]
       __do_sys_sendmsg net/socket.c:2657 [inline]
       __se_sys_sendmsg net/socket.c:2655 [inline]
       __x64_sys_sendmsg+0x19b/0x260 net/socket.c:2655
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #1 (&nbd->config_lock){+.+.}-{4:4}:
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
       refcount_dec_and_mutex_lock+0x30/0xa0 lib/refcount.c:118
       nbd_config_put+0x2c/0x790 drivers/block/nbd.c:1423
       nbd_release+0xfe/0x140 drivers/block/nbd.c:1735
       bdev_release+0x536/0x650 block/bdev.c:-1
       blkdev_release+0x15/0x20 block/fops.c:684
       __fput+0x44c/0xa70 fs/file_table.c:467
       fput_close_sync+0x119/0x200 fs/file_table.c:572
       __do_sys_close fs/open.c:1589 [inline]
       __se_sys_close fs/open.c:1574 [inline]
       __x64_sys_close+0x7f/0x110 fs/open.c:1574
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&disk->open_mutex){+.+.}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3168 [inline]
       check_prevs_add kernel/locking/lockdep.c:3287 [inline]
       validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3911
       __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5240
       lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
       __mutex_lock_common kernel/locking/mutex.c:602 [inline]
       __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
       __del_gendisk+0x129/0x9e0 block/genhd.c:706
       del_gendisk+0xe8/0x160 block/genhd.c:819
       loop_remove+0x42/0xc0 drivers/block/loop.c:2081
       loop_control_remove drivers/block/loop.c:2140 [inline]
       loop_control_ioctl+0x4a6/0x590 drivers/block/loop.c:2178
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:907 [inline]
       __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
       do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
       do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

other info that might help us debug this:

Chain exists of:
  &disk->open_mutex --> &nbd->config_lock --> &set->update_nr_hwq_lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  rlock(&set->update_nr_hwq_lock);
                               lock(&nbd->config_lock);
                               lock(&set->update_nr_hwq_lock);
  lock(&disk->open_mutex);

 *** DEADLOCK ***

1 lock held by syz.4.1558/11720:
 #0: ffff888142bb2368 (&set->update_nr_hwq_lock){++++}-{4:4}, at: del_gendisk+0xe0/0x160 block/genhd.c:818

stack backtrace:
CPU: 0 UID: 0 PID: 11720 Comm: syz.4.1558 Not tainted 6.15.0-rc7-next-20250526-syzkaller #0 PREEMPT(full) 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 05/07/2025
Call Trace:
 <TASK>
 dump_stack_lvl+0x189/0x250 lib/dump_stack.c:120
 print_circular_bug+0x2ee/0x310 kernel/locking/lockdep.c:2046
 check_noncircular+0x134/0x160 kernel/locking/lockdep.c:2178
 check_prev_add kernel/locking/lockdep.c:3168 [inline]
 check_prevs_add kernel/locking/lockdep.c:3287 [inline]
 validate_chain+0xb9b/0x2140 kernel/locking/lockdep.c:3911
 __lock_acquire+0xab9/0xd20 kernel/locking/lockdep.c:5240
 lock_acquire+0x120/0x360 kernel/locking/lockdep.c:5871
 __mutex_lock_common kernel/locking/mutex.c:602 [inline]
 __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:747
 __del_gendisk+0x129/0x9e0 block/genhd.c:706
 del_gendisk+0xe8/0x160 block/genhd.c:819
 loop_remove+0x42/0xc0 drivers/block/loop.c:2081
 loop_control_remove drivers/block/loop.c:2140 [inline]
 loop_control_ioctl+0x4a6/0x590 drivers/block/loop.c:2178
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:907 [inline]
 __se_sys_ioctl+0xf9/0x170 fs/ioctl.c:893
 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
 do_syscall_64+0xfa/0x3b0 arch/x86/entry/syscall_64.c:94
 entry_SYSCALL_64_after_hwframe+0x77/0x7f
RIP: 0033:0x7f301358e969
Code: ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 40 00 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 73 01 c3 48 c7 c1 a8 ff ff ff f7 d8 64 89 01 48
RSP: 002b:00007f30113f6038 EFLAGS: 00000246 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00007f30137b5fa0 RCX: 00007f301358e969
RDX: 0000000000000000 RSI: 0000000000004c81 RDI: 0000000000000003
RBP: 00007f3013610ab1 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
R13: 0000000000000000 R14: 00007f30137b5fa0 R15: 00007fffe9613fd8
 </TASK>


---
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