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BUG: KCSAN: data-race in n_tty_receive_char / start_tty

write to 0xffff888130e9b5bc of 1 bytes by task 5142 on cpu 1:
 __start_tty drivers/tty/tty_io.c:803 [inline]
 start_tty+0x81/0x170 drivers/tty/tty_io.c:825
 n_tty_receive_char_special drivers/tty/n_tty.c:1269 [inline]
 n_tty_receive_buf_standard drivers/tty/n_tty.c:1558 [inline]
 __receive_buf drivers/tty/n_tty.c:1577 [inline]
 n_tty_receive_buf_common+0x107c/0x4e70 drivers/tty/n_tty.c:1674
 n_tty_receive_buf+0x2b/0x40 drivers/tty/n_tty.c:1703
 tiocsti drivers/tty/tty_io.c:2311 [inline]
 tty_ioctl+0x968/0x1120 drivers/tty/tty_io.c:2719
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:1069 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:1055
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:1055
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888130e9b5bc of 1 bytes by task 1794 on cpu 0:
 n_tty_receive_char+0x3d/0x6c0 drivers/tty/n_tty.c:1401
 n_tty_receive_buf_standard drivers/tty/n_tty.c:1560 [inline]
 __receive_buf drivers/tty/n_tty.c:1577 [inline]
 n_tty_receive_buf_common+0xf2e/0x4e70 drivers/tty/n_tty.c:1674
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1709
 tty_ldisc_receive_buf+0x5b/0xf0 drivers/tty/tty_buffer.c:471
 tty_port_default_receive_buf+0x54/0x80 drivers/tty/tty_port.c:39
 receive_buf drivers/tty/tty_buffer.c:491 [inline]
 flush_to_ldisc+0x1c5/0x2c0 drivers/tty/tty_buffer.c:543
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2276
 worker_thread+0x636/0xae0 kernel/workqueue.c:2422
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295

value changed: 0x01 -> 0x00

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 1794 Comm: kworker/u4:6 Tainted: G        W         5.14.0-rc6-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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