syzbot

==================================================================
BUG: KCSAN: data-race in n_hdlc_send_frames / n_hdlc_send_frames

write to 0xffff88809882ed1c of 4 bytes by task 11896 on cpu 1:
 n_hdlc_send_frames+0xdd/0x340 drivers/tty/n_hdlc.c:392
 n_hdlc_tty_write+0x38d/0x3b0 drivers/tty/n_hdlc.c:713
 do_tty_write drivers/tty/tty_io.c:962 [inline]
 tty_write+0x2fb/0x4e0 drivers/tty/tty_io.c:1046
 __vfs_write+0x67/0xc0 fs/read_write.c:494
 vfs_write fs/read_write.c:558 [inline]
 vfs_write+0x18a/0x390 fs/read_write.c:542
 ksys_write+0xd5/0x1b0 fs/read_write.c:611
 __do_sys_write fs/read_write.c:623 [inline]
 __se_sys_write fs/read_write.c:620 [inline]
 __x64_sys_write+0x4c/0x60 fs/read_write.c:620
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88809882ed1c of 4 bytes by task 11899 on cpu 0:
 n_hdlc_send_frames+0x276/0x340 drivers/tty/n_hdlc.c:450
 n_hdlc_tty_wakeup+0xa5/0xc0 drivers/tty/n_hdlc.c:479
 tty_ldisc_hangup+0x225/0x360 drivers/tty/tty_ldisc.c:730
 __tty_hangup+0x2d8/0x460 drivers/tty/tty_io.c:625
 tty_vhangup drivers/tty/tty_io.c:698 [inline]
 tty_ioctl+0x3af/0xe10 drivers/tty/tty_io.c:2601
 vfs_ioctl fs/ioctl.c:47 [inline]
 file_ioctl fs/ioctl.c:545 [inline]
 do_vfs_ioctl+0x84f/0xcf0 fs/ioctl.c:732
 ksys_ioctl+0xbd/0xe0 fs/ioctl.c:749
 __do_sys_ioctl fs/ioctl.c:756 [inline]
 __se_sys_ioctl fs/ioctl.c:754 [inline]
 __x64_sys_ioctl+0x4c/0x60 fs/ioctl.c:754
 do_syscall_64+0xcc/0x3a0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 11899 Comm: syz-executor.2 Not tainted 5.5.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
==================================================================