======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/1:4/4232 is trying to acquire lock:
ffff88802bd4c2e8 (&tty->termios_rwsem){++++}-{3:3}, at: n_tty_flush_buffer+0x2c/0x1e0 drivers/tty/n_tty.c:369

but task is already holding lock:
ffff88802bbae0b8 (&buf->lock){+.+.}-{3:3}, at: tty_buffer_flush+0x78/0x370 drivers/tty/tty_buffer.c:230

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&buf->lock){+.+.}-{3:3}:
       __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
       __mutex_lock kernel/locking/mutex.c:729 [inline]
       mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
       tty_buffer_flush+0x78/0x370 drivers/tty/tty_buffer.c:230
       tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:400
       __do_SAK+0x260/0x5f0 drivers/tty/tty_io.c:3051
       vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:993
       process_one_work+0x867/0xff0 kernel/workqueue.c:2310
       worker_thread+0xad7/0x12a0 kernel/workqueue.c:2457
       kthread+0x42e/0x520 kernel/kthread.c:334
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287

-> #2 (console_lock){+.+.}-{0:0}:
       console_lock+0x47/0x70 kernel/printk/printk.c:2557
       uart_configure_port drivers/tty/serial/serial_core.c:2425 [inline]
       uart_add_one_port+0x826/0x1d10 drivers/tty/serial/serial_core.c:2980
       serial8250_register_8250_port+0x137f/0x1a80 drivers/tty/serial/8250/8250_core.c:1094
       serial_pnp_probe+0x569/0x740 drivers/tty/serial/8250/8250_pnp.c:482
       pnp_device_probe+0x2bc/0x470 drivers/pnp/driver.c:111
       call_driver_probe drivers/base/dd.c:-1 [inline]
       really_probe+0x276/0xc70 drivers/base/dd.c:595
       __driver_probe_device+0x1f5/0x390 drivers/base/dd.c:775
       driver_probe_device+0x4f/0x420 drivers/base/dd.c:805
       __driver_attach+0x45d/0x660 drivers/base/dd.c:1184
       bus_for_each_dev+0x182/0x1f0 drivers/base/bus.c:303
       bus_add_driver+0x324/0x5d0 drivers/base/bus.c:620
       driver_register+0x32d/0x430 drivers/base/driver.c:240
       serial8250_init+0x96/0x170 drivers/tty/serial/8250/8250_core.c:1190
       do_one_initcall+0x272/0x730 init/main.c:1315
       do_initcall_level+0x13d/0x1f0 init/main.c:1388
       do_initcalls+0x4b/0x90 init/main.c:1404
       kernel_init_freeable+0x3d5/0x560 init/main.c:1628
       kernel_init+0x19/0x1b0 init/main.c:1519
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287

-> #1 (&port->mutex){+.+.}-{3:3}:
       __mutex_lock_common+0x1e5/0x2400 kernel/locking/mutex.c:596
       __mutex_lock kernel/locking/mutex.c:729 [inline]
       mutex_lock_nested+0x17/0x20 kernel/locking/mutex.c:743
       uart_set_termios+0x80/0x900 drivers/tty/serial/serial_core.c:1522
       tty_set_termios+0xc73/0x1770 drivers/tty/tty_ioctl.c:358
       set_termios+0x6d3/0x8a0 drivers/tty/tty_ioctl.c:458
       tty_mode_ioctl+0x686/0x890 drivers/tty/tty_ioctl.c:-1
       tty_ioctl+0x8e3/0xc30 drivers/tty/tty_io.c:2819
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:874 [inline]
       __se_sys_ioctl+0xfa/0x170 fs/ioctl.c:860
       do_syscall_x64 arch/x86/entry/common.c:50 [inline]
       do_syscall_64+0x4c/0xa0 arch/x86/entry/common.c:80
       entry_SYSCALL_64_after_hwframe+0x66/0xd0

-> #0 (&tty->termios_rwsem){++++}-{3:3}:
       check_prev_add kernel/locking/lockdep.c:3053 [inline]
       check_prevs_add kernel/locking/lockdep.c:3172 [inline]
       validate_chain kernel/locking/lockdep.c:3788 [inline]
       __lock_acquire+0x2c66/0x7b50 kernel/locking/lockdep.c:5012
       lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5623
       down_write+0x38/0x60 kernel/locking/rwsem.c:1551
       n_tty_flush_buffer+0x2c/0x1e0 drivers/tty/n_tty.c:369
       tty_buffer_flush+0x2e4/0x370 drivers/tty/tty_buffer.c:241
       tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:400
       __do_SAK+0x260/0x5f0 drivers/tty/tty_io.c:3051
       vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:993
       process_one_work+0x867/0xff0 kernel/workqueue.c:2310
       worker_thread+0xad7/0x12a0 kernel/workqueue.c:2457
       kthread+0x42e/0x520 kernel/kthread.c:334
       ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287

other info that might help us debug this:

Chain exists of:
  &tty->termios_rwsem --> console_lock --> &buf->lock

 Possible unsafe locking scenario:

       CPU0                    CPU1
       ----                    ----
  lock(&buf->lock);
                               lock(console_lock);
                               lock(&buf->lock);
  lock(&tty->termios_rwsem);

 *** DEADLOCK ***

5 locks held by kworker/1:4/4232:
 #0: ffff888016c70938 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x75c/0xff0 kernel/workqueue.c:-1
 #1: ffffc9000313fd00 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_one_work+0x79e/0xff0 kernel/workqueue.c:2285
 #2: ffffffff8c313aa0 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x24/0x210 drivers/tty/vt/vt_ioctl.c:983
 #3: ffff88802bd4c098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref drivers/tty/tty_ldisc.c:273 [inline]
 #3: ffff88802bd4c098 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_flush+0x1c/0xc0 drivers/tty/tty_ldisc.c:398
 #4: ffff88802bbae0b8 (&buf->lock){+.+.}-{3:3}, at: tty_buffer_flush+0x78/0x370 drivers/tty/tty_buffer.c:230

stack backtrace:
CPU: 1 PID: 4232 Comm: kworker/1:4 Not tainted syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Workqueue: events vc_SAK
Call Trace:
 <TASK>
 dump_stack_lvl+0x188/0x250 lib/dump_stack.c:106
 check_noncircular+0x296/0x330 kernel/locking/lockdep.c:2133
 check_prev_add kernel/locking/lockdep.c:3053 [inline]
 check_prevs_add kernel/locking/lockdep.c:3172 [inline]
 validate_chain kernel/locking/lockdep.c:3788 [inline]
 __lock_acquire+0x2c66/0x7b50 kernel/locking/lockdep.c:5012
 lock_acquire+0x19e/0x400 kernel/locking/lockdep.c:5623
 down_write+0x38/0x60 kernel/locking/rwsem.c:1551
 n_tty_flush_buffer+0x2c/0x1e0 drivers/tty/n_tty.c:369
 tty_buffer_flush+0x2e4/0x370 drivers/tty/tty_buffer.c:241
 tty_ldisc_flush+0x6a/0xc0 drivers/tty/tty_ldisc.c:400
 __do_SAK+0x260/0x5f0 drivers/tty/tty_io.c:3051
 vc_SAK+0x74/0x210 drivers/tty/vt/vt_ioctl.c:993
 process_one_work+0x867/0xff0 kernel/workqueue.c:2310
 worker_thread+0xad7/0x12a0 kernel/workqueue.c:2457
 kthread+0x42e/0x520 kernel/kthread.c:334
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:287
 </TASK>
Bluetooth: hci0: command 0x0419 tx timeout
Bluetooth: hci1: command 0x0419 tx timeout
Bluetooth: hci3: command 0x0419 tx timeout
Bluetooth: hci4: command 0x0419 tx timeout