* Re: [syzbot] [kernel?^W NDB] possible deadlock in worker_thread (3)
[not found] <69988ebc.a70a0220.2c38d7.0142.GAE@google.com>
@ 2026-02-22 21:46 ` Thomas Gleixner
0 siblings, 0 replies; only message in thread
From: Thomas Gleixner @ 2026-02-22 21:46 UTC (permalink / raw)
To: syzbot, bp, dave.hansen, hpa, linux-kernel, mingo, syzkaller-bugs,
x86
Cc: linux-block, Josef Bacik, nbd
On Fri, Feb 20 2026 at 08:41, syzbot wrote:
> Hello,
>
> syzbot found the following issue on:
>
> HEAD commit: 635c467cc14e Add linux-next specific files for 20260213
> git tree: linux-next
> console output: https://syzkaller.appspot.com/x/log.txt?x=15339b3a580000
> kernel config: https://syzkaller.appspot.com/x/.config?x=61690c38d1398936
> dashboard link: https://syzkaller.appspot.com/bug?extid=0b6ec149bb8b98bd9485
> compiler: Debian clang version 21.1.8 (++20251221033036+2078da43e25a-1~exp1~20251221153213.50), Debian LLD 21.1.8
>
> Unfortunately, I don't have any reproducer for this issue yet.
>
> Downloadable assets:
> disk image: https://storage.googleapis.com/syzbot-assets/78b3d15ca8e6/disk-635c467c.raw.xz
> vmlinux: https://storage.googleapis.com/syzbot-assets/a95f3d108ef4/vmlinux-635c467c.xz
> kernel image: https://storage.googleapis.com/syzbot-assets/e58086838b24/bzImage-635c467c.xz
>
> IMPORTANT: if you fix the issue, please add the following tag to the commit:
> Reported-by: syzbot+0b6ec149bb8b98bd9485@syzkaller.appspotmail.com
>
> block nbd1: Receive control failed (result -32)
> ======================================================
> WARNING: possible circular locking dependency detected
> syzkaller #0 Not tainted
> ------------------------------------------------------
> kworker/u9:3/5827 is trying to acquire lock:
> ffff8880275b9a78 (&nbd->config_lock){+.+.}-{4:4}, at: refcount_dec_and_mutex_lock+0x30/0xa0 lib/refcount.c:118
>
> but task is already holding lock:
> ffffc90003bd7c40 ((work_completion)(&args->work)#3){+.+.}-{0:0}, at: process_one_work+0x87c/0x1650 kernel/workqueue.c:3255
>
> which lock already depends on the new lock.
This is clearly a NDB issue and the lockdep splat is pretty clear.
Cc'ed the maintainer and left the report intact.
>
> the existing dependency chain (in reverse order) is:
>
> -> #2 ((work_completion)(&args->work)#3){+.+.}-{0:0}:
> process_one_work+0x895/0x1650 kernel/workqueue.c:3255
> process_scheduled_works kernel/workqueue.c:3362 [inline]
> worker_thread+0xb46/0x1140 kernel/workqueue.c:3443
> kthread+0x388/0x470 kernel/kthread.c:467
> ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> -> #1 ((wq_completion)nbd3-recv){+.+.}-{0:0}:
> touch_wq_lockdep_map+0xcb/0x180 kernel/workqueue.c:3994
> __flush_workqueue+0x14b/0x14f0 kernel/workqueue.c:4036
> nbd_disconnect_and_put+0x9e/0x2c0 drivers/block/nbd.c:2264
> nbd_genl_disconnect+0x4a9/0x590 drivers/block/nbd.c:2303
> genl_family_rcv_msg_doit+0x22a/0x330 net/netlink/genetlink.c:1115
> genl_family_rcv_msg net/netlink/genetlink.c:1195 [inline]
> genl_rcv_msg+0x61c/0x7a0 net/netlink/genetlink.c:1210
> netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
> genl_rcv+0x28/0x40 net/netlink/genetlink.c:1219
> netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
> netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
> netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
> sock_sendmsg_nosec+0x18f/0x1d0 net/socket.c:737
> __sock_sendmsg net/socket.c:752 [inline]
> ____sys_sendmsg+0x589/0x8c0 net/socket.c:2610
> ___sys_sendmsg+0x2a5/0x360 net/socket.c:2664
> __sys_sendmsg net/socket.c:2696 [inline]
> __do_sys_sendmsg net/socket.c:2701 [inline]
> __se_sys_sendmsg net/socket.c:2699 [inline]
> __x64_sys_sendmsg+0x1bd/0x2a0 net/socket.c:2699
> do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline]
> do_syscall_64+0x14d/0xf80 arch/x86/entry/syscall_64.c:94
> entry_SYSCALL_64_after_hwframe+0x77/0x7f
>
> -> #0 (&nbd->config_lock){+.+.}-{4:4}:
> check_prev_add kernel/locking/lockdep.c:3165 [inline]
> check_prevs_add kernel/locking/lockdep.c:3284 [inline]
> validate_chain kernel/locking/lockdep.c:3908 [inline]
> __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
> lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
> __mutex_lock_common kernel/locking/mutex.c:614 [inline]
> __mutex_lock+0x19f/0x1300 kernel/locking/mutex.c:776
> refcount_dec_and_mutex_lock+0x30/0xa0 lib/refcount.c:118
> nbd_config_put+0x2c/0x580 drivers/block/nbd.c:1434
> recv_work+0x1cc1/0x1d90 drivers/block/nbd.c:1026
> process_one_work+0x949/0x1650 kernel/workqueue.c:3279
> process_scheduled_works kernel/workqueue.c:3362 [inline]
> worker_thread+0xb46/0x1140 kernel/workqueue.c:3443
> kthread+0x388/0x470 kernel/kthread.c:467
> ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
>
> other info that might help us debug this:
>
> Chain exists of:
> &nbd->config_lock --> (wq_completion)nbd3-recv --> (work_completion)(&args->work)#3
>
> Possible unsafe locking scenario:
>
> CPU0 CPU1
> ---- ----
> lock((work_completion)(&args->work)#3);
> lock((wq_completion)nbd3-recv);
> lock((work_completion)(&args->work)#3);
> lock(&nbd->config_lock);
>
> *** DEADLOCK ***
>
> 2 locks held by kworker/u9:3/5827:
> #0: ffff8880275c9148 ((wq_completion)nbd1-recv){+.+.}-{0:0}, at: process_one_work+0x855/0x1650 kernel/workqueue.c:3254
> #1: ffffc90003bd7c40 ((work_completion)(&args->work)#3){+.+.}-{0:0}, at: process_one_work+0x87c/0x1650 kernel/workqueue.c:3255
>
> stack backtrace:
> CPU: 0 UID: 0 PID: 5827 Comm: kworker/u9:3 Not tainted syzkaller #0 PREEMPT(full)
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 02/12/2026
> Workqueue: nbd1-recv recv_work
> Call Trace:
> <TASK>
> dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
> print_circular_bug+0x2e1/0x300 kernel/locking/lockdep.c:2043
> check_noncircular+0x12e/0x150 kernel/locking/lockdep.c:2175
> check_prev_add kernel/locking/lockdep.c:3165 [inline]
> check_prevs_add kernel/locking/lockdep.c:3284 [inline]
> validate_chain kernel/locking/lockdep.c:3908 [inline]
> __lock_acquire+0x15a5/0x2cf0 kernel/locking/lockdep.c:5237
> lock_acquire+0xf0/0x2e0 kernel/locking/lockdep.c:5868
> __mutex_lock_common kernel/locking/mutex.c:614 [inline]
> __mutex_lock+0x19f/0x1300 kernel/locking/mutex.c:776
> refcount_dec_and_mutex_lock+0x30/0xa0 lib/refcount.c:118
> nbd_config_put+0x2c/0x580 drivers/block/nbd.c:1434
> recv_work+0x1cc1/0x1d90 drivers/block/nbd.c:1026
> process_one_work+0x949/0x1650 kernel/workqueue.c:3279
> process_scheduled_works kernel/workqueue.c:3362 [inline]
> worker_thread+0xb46/0x1140 kernel/workqueue.c:3443
> kthread+0x388/0x470 kernel/kthread.c:467
> ret_from_fork+0x51e/0xb90 arch/x86/kernel/process.c:158
> ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
> </TASK>
> block nbd1: shutting down sockets
>
>
> ---
> This report is generated by a bot. It may contain errors.
> See https://goo.gl/tpsmEJ for more information about syzbot.
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>
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>
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