batman_adv: batadv0: Interface activated: batadv_slave_1
IPv6: ADDRCONF(NETDEV_CHANGE): batadv_slave_1: link becomes ready
IPv6: ADDRCONF(NETDEV_CHANGE): veth1_to_batadv: link becomes ready
============================================
WARNING: possible recursive locking detected
4.19.107-syzkaller #0 Not tainted
--------------------------------------------
swapper/0/0 is trying to acquire lock:
0000000053c8769e (&port_lock_key){-.-.}, at: uart_write+0x1c7/0x680 drivers/tty/serial/serial_core.c:591

but task is already holding lock:
0000000053c8769e (&port_lock_key){-.-.}, at: serial8250_handle_irq.part.0+0x21/0x290 drivers/tty/serial/8250/8250_port.c:1868

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(&port_lock_key);
  lock(&port_lock_key);

 *** DEADLOCK ***

 May be due to missing lock nesting notation

3 locks held by swapper/0/0:
 #0: 0000000058c89ec8 (&(&i->lock)->rlock){-.-.}, at: spin_lock include/linux/spinlock.h:329 [inline]
 #0: 0000000058c89ec8 (&(&i->lock)->rlock){-.-.}, at: serial8250_interrupt+0x2b/0x1d0 drivers/tty/serial/8250/8250_core.c:115
 #1: 0000000053c8769e (&port_lock_key){-.-.}, at: serial8250_handle_irq.part.0+0x21/0x290 drivers/tty/serial/8250/8250_port.c:1868
 #2: 000000003d674e2f (&tty->ldisc_sem){++++}, at: tty_ldisc_ref+0x1d/0x80 drivers/tty/tty_ldisc.c:293

stack backtrace:
CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.19.107-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Call Trace:
 <IRQ>
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 print_deadlock_bug kernel/locking/lockdep.c:1759 [inline]
 check_deadlock kernel/locking/lockdep.c:1803 [inline]
 validate_chain kernel/locking/lockdep.c:2399 [inline]
 __lock_acquire.cold+0x20f/0x4a7 kernel/locking/lockdep.c:3411
 lock_acquire+0x170/0x400 kernel/locking/lockdep.c:3903
 __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
 _raw_spin_lock_irqsave+0x8c/0xbf kernel/locking/spinlock.c:152
 uart_write+0x1c7/0x680 drivers/tty/serial/serial_core.c:591
 n_hdlc_send_frames+0x276/0x450 drivers/tty/n_hdlc.c:403
 n_hdlc_tty_wakeup+0xa0/0xc0 drivers/tty/n_hdlc.c:479
 tty_wakeup+0xd4/0x110 drivers/tty/tty_io.c:534
 tty_port_default_wakeup+0x26/0x40 drivers/tty/tty_port.c:50
 serial8250_tx_chars+0x48f/0xae0 drivers/tty/serial/8250/8250_port.c:1806
 serial8250_handle_irq.part.0+0x24b/0x290 drivers/tty/serial/8250/8250_port.c:1879
 serial8250_handle_irq drivers/tty/serial/8250/8250_port.c:1865 [inline]
 serial8250_default_handle_irq+0xb5/0x140 drivers/tty/serial/8250/8250_port.c:1895
 serial8250_interrupt+0xf2/0x1d0 drivers/tty/serial/8250/8250_core.c:125
 __handle_irq_event_percpu+0x144/0x8e0 kernel/irq/handle.c:149
 handle_irq_event_percpu+0x76/0x160 kernel/irq/handle.c:189
 handle_irq_event+0xa2/0x12d kernel/irq/handle.c:206
 handle_edge_irq+0x24b/0x8c0 kernel/irq/chip.c:797
 generic_handle_irq_desc include/linux/irqdesc.h:155 [inline]
 handle_irq+0x35/0x50 arch/x86/kernel/irq_64.c:87
 do_IRQ+0x93/0x1c0 arch/x86/kernel/irq.c:246
 common_interrupt+0xf/0xf arch/x86/entry/entry_64.S:670
 </IRQ>
RIP: 0010:native_safe_halt+0xe/0x10 arch/x86/include/asm/irqflags.h:61
Code: fa eb 82 90 90 90 90 90 90 90 90 90 e9 07 00 00 00 0f 00 2d f4 02 48 00 f4 c3 66 90 e9 07 00 00 00 0f 00 2d e4 02 48 00 fb f4 <c3> 90 41 56 41 55 41 54 55 53 e8 23 c2 36 fa e8 ce ce 35 fc 0f 1f
RSP: 0018:ffffffff88a07d40 EFLAGS: 00000282 ORIG_RAX: ffffffffffffffd7
RAX: 1ffffffff1164ad4 RBX: dffffc0000000000 RCX: 0000000000000000
RDX: dffffc0000000000 RSI: 0000000000000001 RDI: ffffffff88a7a73c
RBP: 0000000000000000 R08: ffffffff88a79ec0 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: ffffffff88b25690
R13: 0000000000000000 R14: 0000000000000000 R15: ffffffff88a79ec0
 arch_safe_halt arch/x86/include/asm/paravirt.h:94 [inline]
 default_idle+0x49/0x320 arch/x86/kernel/process.c:565
 cpuidle_idle_call kernel/sched/idle.c:153 [inline]
 do_idle+0x2ee/0x4b0 kernel/sched/idle.c:263
 cpu_startup_entry+0xc6/0xd0 kernel/sched/idle.c:369
 start_kernel+0x7e4/0x81c init/main.c:737
 secondary_startup_64+0xa4/0xb0 arch/x86/kernel/head_64.S:243