Re: [syzbot] [block?] possible deadlock in __del_gendisk

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

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
possible deadlock in __del_gendisk

======================================================
WARNING: possible circular locking dependency detected
6.15.0-syzkaller-g90b83efa6701-dirty #0 Not tainted
------------------------------------------------------
kworker/u4:2/31 is trying to acquire lock:
ffff888000617358 (&disk->open_mutex){+.+.}-{4:4}, at: __del_gendisk+0x129/0x9e0 block/genhd.c:706

but task is already holding lock:
ffff88803f055188 (&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:2203
       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:601 [inline]
       __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:746
       nbd_open+0x2c3/0x5f0 drivers/block/nbd.c:1704
       blkdev_get_whole+0x98/0x510 block/bdev.c:730
       bdev_open+0x31e/0xd30 block/bdev.c:957
       blkdev_open+0x3a8/0x510 block/fops.c:676
       do_dentry_open+0xdf0/0x1970 fs/open.c:964
       vfs_open+0x3b/0x340 fs/open.c:1094
       do_open fs/namei.c:3887 [inline]
       path_openat+0x2ee5/0x3830 fs/namei.c:4046
       do_filp_open+0x1fa/0x410 fs/namei.c:4073
       do_sys_openat2+0x121/0x1c0 fs/open.c:1437
       do_sys_open fs/open.c:1452 [inline]
       __do_sys_openat fs/open.c:1468 [inline]
       __se_sys_openat fs/open.c:1463 [inline]
       __x64_sys_openat+0x138/0x170 fs/open.c:1463
       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:601 [inline]
       __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:746
       __del_gendisk+0x129/0x9e0 block/genhd.c:706
       del_gendisk+0xe8/0x160 block/genhd.c:819
       nbd_dev_remove drivers/block/nbd.c:268 [inline]
       nbd_dev_remove_work+0x47/0xe0 drivers/block/nbd.c:284
       process_one_work kernel/workqueue.c:3238 [inline]
       process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3321
       worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
       kthread+0x70e/0x8a0 kernel/kthread.c:464
       ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

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 ***

3 locks held by kworker/u4:2/31:
 #0: ffff88801f97e148 ((wq_completion)nbd-del){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3213 [inline]
 #0: ffff88801f97e148 ((wq_completion)nbd-del){+.+.}-{0:0}, at: process_scheduled_works+0x9b4/0x17b0 kernel/workqueue.c:3321
 #1: ffffc90000527bc0 ((work_completion)(&nbd->remove_work)){+.+.}-{0:0}, at: process_one_work kernel/workqueue.c:3214 [inline]
 #1: ffffc90000527bc0 ((work_completion)(&nbd->remove_work)){+.+.}-{0:0}, at: process_scheduled_works+0x9ef/0x17b0 kernel/workqueue.c:3321
 #2: ffff88803f055188 (&set->update_nr_hwq_lock){++++}-{4:4}, at: del_gendisk+0xe0/0x160 block/genhd.c:818

stack backtrace:
CPU: 0 UID: 0 PID: 31 Comm: kworker/u4:2 Not tainted 6.15.0-syzkaller-g90b83efa6701-dirty #0 PREEMPT(full) 
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.16.3-debian-1.16.3-2~bpo12+1 04/01/2014
Workqueue: nbd-del nbd_dev_remove_work
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:601 [inline]
 __mutex_lock+0x182/0xe80 kernel/locking/mutex.c:746
 __del_gendisk+0x129/0x9e0 block/genhd.c:706
 del_gendisk+0xe8/0x160 block/genhd.c:819
 nbd_dev_remove drivers/block/nbd.c:268 [inline]
 nbd_dev_remove_work+0x47/0xe0 drivers/block/nbd.c:284
 process_one_work kernel/workqueue.c:3238 [inline]
 process_scheduled_works+0xae1/0x17b0 kernel/workqueue.c:3321
 worker_thread+0x8a0/0xda0 kernel/workqueue.c:3402
 kthread+0x70e/0x8a0 kernel/kthread.c:464
 ret_from_fork+0x3fc/0x770 arch/x86/kernel/process.c:148
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>


Tested on:

commit:         90b83efa Merge tag 'bpf-next-6.16' of git://git.kernel..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=16892482580000
kernel config:  https://syzkaller.appspot.com/x/.config?x=262b2977ef00756b
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
patch:          https://syzkaller.appspot.com/x/patch.diff?x=1345dbf4580000