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BUG: KCSAN: data-race in n_hdlc_buf_get / n_hdlc_tty_receive

write to 0xffff8881200a2e48 of 4 bytes by task 12903 on cpu 0:
 n_hdlc_buf_get+0x132/0x180 drivers/tty/n_hdlc.c:934
 flush_rx_queue+0x50/0x70 drivers/tty/n_hdlc.c:199
 tty_ldisc_hangup+0x5e/0x340 drivers/tty/tty_ldisc.c:726
 __tty_hangup+0x2cb/0x440 drivers/tty/tty_io.c:625
 tty_vhangup drivers/tty/tty_io.c:698 [inline]
 tty_ioctl+0x395/0xd80 drivers/tty/tty_io.c:2601
 vfs_ioctl fs/ioctl.c:47 [inline]
 ksys_ioctl+0x101/0x150 fs/ioctl.c:763
 __do_sys_ioctl fs/ioctl.c:772 [inline]
 __se_sys_ioctl fs/ioctl.c:770 [inline]
 __x64_sys_ioctl+0x47/0x60 fs/ioctl.c:770
 do_syscall_64+0xc7/0x390 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff8881200a2e48 of 4 bytes by task 654 on cpu 1:
 n_hdlc_tty_receive+0x1cf/0x2a0 drivers/tty/n_hdlc.c:524
 tty_ldisc_receive_buf+0xde/0xe0 drivers/tty/tty_buffer.c:465
 tty_port_default_receive_buf+0x82/0xd0 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1dd/0x260 drivers/tty/tty_buffer.c:533
 process_one_work+0x424/0x930 kernel/workqueue.c:2264
 worker_thread+0x9a/0x7e0 kernel/workqueue.c:2410
 kthread+0x1cb/0x1f0 kernel/kthread.c:255
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 654 Comm: kworker/u4:4 Not tainted 5.6.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: events_unbound flush_to_ldisc
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