======================================================
WARNING: possible circular locking dependency detected
syzkaller #0 Not tainted
------------------------------------------------------
kworker/0:1/10 is trying to acquire lock:
ffff88803718f2a8 (&tty->termios_rwsem){++++}-{4:4}, at: class_rwsem_write_constructor include/linux/rwsem.h:270 [inline]
ffff88803718f2a8 (&tty->termios_rwsem){++++}-{4:4}, at: n_tty_flush_buffer+0x2f/0x220 drivers/tty/n_tty.c:349

but task is already holding lock:
ffff88801aea80b0 (&buf->lock){+.+.}-{4:4}, at: tty_buffer_flush+0x74/0x2b0 drivers/tty/tty_buffer.c:236

which lock already depends on the new lock.


the existing dependency chain (in reverse order) is:

-> #3 (&buf->lock){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:559 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:578
       tty_buffer_flush+0x74/0x2b0 drivers/tty/tty_buffer.c:236
       tty_ldisc_flush+0x6b/0xc0 drivers/tty/tty_ldisc.c:388
       __do_SAK+0x254/0x680 drivers/tty/tty_io.c:3025
       vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:994
       process_one_work+0xaaf/0x1480 kernel/workqueue.c:3379
       process_scheduled_works kernel/workqueue.c:3462 [inline]
       worker_thread+0xb05/0x10d0 kernel/workqueue.c:3543
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #2 (console_lock){+.+.}-{0:0}:
       console_lock+0x76/0x90 kernel/printk/printk.c:2896
       uart_configure_port drivers/tty/serial/serial_core.c:2530 [inline]
       serial_core_add_one_port drivers/tty/serial/serial_core.c:3101 [inline]
       serial_core_register_port+0xc47/0x2840 drivers/tty/serial/serial_core.c:3322
       serial8250_register_8250_port+0x16e1/0x20c0 drivers/tty/serial/8250/8250_core.c:828
       serial_pnp_probe+0x570/0x800 drivers/tty/serial/8250/8250_pnp.c:480
       pnp_device_probe+0x30b/0x4c0 drivers/pnp/driver.c:111
       call_driver_probe drivers/base/dd.c:-1 [inline]
       really_probe+0x254/0xae0 drivers/base/dd.c:706
       __driver_probe_device+0x1e8/0x360 drivers/base/dd.c:868
       driver_probe_device+0x4f/0x240 drivers/base/dd.c:898
       __driver_attach+0x339/0x600 drivers/base/dd.c:1292
       bus_for_each_dev+0x23e/0x2c0 drivers/base/bus.c:383
       bus_add_driver+0x348/0x670 drivers/base/bus.c:763
       driver_register+0x23a/0x320 drivers/base/driver.c:174
       serial8250_init+0x8f/0x160 drivers/tty/serial/8250/8250_platform.c:317
       do_one_initcall+0x250/0x870 init/main.c:1357
       do_initcall_level+0x10a/0x1a0 init/main.c:1419
       do_initcalls+0x59/0xa0 init/main.c:1435
       kernel_init_freeable+0x29d/0x3e0 init/main.c:1670
       kernel_init+0x22/0x1d0 init/main.c:1560
       ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
       ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245

-> #1 (&port->mutex){+.+.}-{4:4}:
       __mutex_lock_common kernel/locking/rtmutex_api.c:559 [inline]
       mutex_lock_nested+0x5a/0x1d0 kernel/locking/rtmutex_api.c:578
       class_mutex_constructor include/linux/mutex.h:253 [inline]
       uart_set_termios+0x84/0x12e0 drivers/tty/serial/serial_core.c:1645
       tty_set_termios+0xcaf/0x1800 drivers/tty/tty_ioctl.c:341
       set_termios+0x53d/0x6c0 drivers/tty/tty_ioctl.c:516
       tty_mode_ioctl+0x49c/0x7c0 drivers/tty/tty_ioctl.c:-1
       tty_ioctl+0x9c3/0xde0 drivers/tty/tty_io.c:2801
       vfs_ioctl fs/ioctl.c:51 [inline]
       __do_sys_ioctl fs/ioctl.c:597 [inline]
       __se_sys_ioctl+0xff/0x170 fs/ioctl.c:583
       do_syscall_x64 arch/x86/entry/syscall_64.c:61 [inline]
       do_syscall_64+0x17b/0x530 arch/x86/entry/syscall_64.c:85
       entry_SYSCALL_64_after_hwframe+0x77/0x7f

-> #0 (&tty->termios_rwsem){++++}-{4:4}:
       check_prev_add kernel/locking/lockdep.c:3181 [inline]
       check_prevs_add kernel/locking/lockdep.c:3300 [inline]
       validate_chain kernel/locking/lockdep.c:3924 [inline]
       __lock_acquire+0x15ff/0x2e40 kernel/locking/lockdep.c:5254
       lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5906
       down_write+0x3a/0x50 kernel/locking/rwsem.c:1631
       class_rwsem_write_constructor include/linux/rwsem.h:270 [inline]
       n_tty_flush_buffer+0x2f/0x220 drivers/tty/n_tty.c:349
       tty_buffer_flush+0x1ec/0x2b0 drivers/tty/tty_buffer.c:248
       tty_ldisc_flush+0x6b/0xc0 drivers/tty/tty_ldisc.c:388
       __do_SAK+0x254/0x680 drivers/tty/tty_io.c:3025
       vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:994
       process_one_work+0xaaf/0x1480 kernel/workqueue.c:3379
       process_scheduled_works kernel/workqueue.c:3462 [inline]
       worker_thread+0xb05/0x10d0 kernel/workqueue.c:3543
       kthread+0x388/0x470 kernel/kthread.c:436
       ret_from_fork+0x514/0xb70 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:
  &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 ***

locks held by kworker/0:1/10: 5, last CPU#0:
 #0: ffff88813fe63938 ((wq_completion)events){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
 #0: ffff88813fe63938 ((wq_completion)events){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
 #0: ffff88813fe63938 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0x7fd/0x1480 kernel/workqueue.c:3344
 #1: ffffc900000f7c60 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: rcu_lock_acquire include/linux/rcupdate.h:300 [inline]
 #1: ffffc900000f7c60 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: rcu_read_lock include/linux/rcupdate.h:840 [inline]
 #1: ffffc900000f7c60 ((work_completion)(&vc_cons[currcons].SAK_work)){+.+.}-{0:0}, at: process_one_work+0x7fd/0x1480 kernel/workqueue.c:3344
 #2: ffffffff8e3b5a40 (console_lock){+.+.}-{0:0}, at: class_console_lock_constructor include/linux/console.h:734 [inline]
 #2: ffffffff8e3b5a40 (console_lock){+.+.}-{0:0}, at: vc_SAK+0x28/0x220 drivers/tty/vt/vt_ioctl.c:985
 #3: ffff88803718f0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_ref drivers/tty/tty_ldisc.c:263 [inline]
 #3: ffff88803718f0a0 (&tty->ldisc_sem){++++}-{0:0}, at: tty_ldisc_flush+0x20/0xc0 drivers/tty/tty_ldisc.c:386
 #4: ffff88801aea80b0 (&buf->lock){+.+.}-{4:4}, at: tty_buffer_flush+0x74/0x2b0 drivers/tty/tty_buffer.c:236

stack backtrace:
CPU: 0 UID: 0 PID: 10 Comm: kworker/0:1 Not tainted syzkaller #0 PREEMPT_{RT,(full)} 
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 07/24/2026
Workqueue: events vc_SAK
Call Trace:
 <TASK>
 dump_stack_lvl+0xe8/0x150 lib/dump_stack.c:120
 print_circular_bug+0x2e2/0x300 kernel/locking/lockdep.c:2059
 check_noncircular+0x12f/0x150 kernel/locking/lockdep.c:2191
 check_prev_add kernel/locking/lockdep.c:3181 [inline]
 check_prevs_add kernel/locking/lockdep.c:3300 [inline]
 validate_chain kernel/locking/lockdep.c:3924 [inline]
 __lock_acquire+0x15ff/0x2e40 kernel/locking/lockdep.c:5254
 lock_acquire+0x115/0x350 kernel/locking/lockdep.c:5906
 down_write+0x3a/0x50 kernel/locking/rwsem.c:1631
 class_rwsem_write_constructor include/linux/rwsem.h:270 [inline]
 n_tty_flush_buffer+0x2f/0x220 drivers/tty/n_tty.c:349
 tty_buffer_flush+0x1ec/0x2b0 drivers/tty/tty_buffer.c:248
 tty_ldisc_flush+0x6b/0xc0 drivers/tty/tty_ldisc.c:388
 __do_SAK+0x254/0x680 drivers/tty/tty_io.c:3025
 vc_SAK+0x78/0x220 drivers/tty/vt/vt_ioctl.c:994
 process_one_work+0xaaf/0x1480 kernel/workqueue.c:3379
 process_scheduled_works kernel/workqueue.c:3462 [inline]
 worker_thread+0xb05/0x10d0 kernel/workqueue.c:3543
 kthread+0x388/0x470 kernel/kthread.c:436
 ret_from_fork+0x514/0xb70 arch/x86/kernel/process.c:158
 ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:245
 </TASK>
usb 6-1: USB disconnect, device number 3