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BUG: KCSAN: data-race in n_tty_receive_buf_common / n_tty_receive_buf_common

write to 0xffffc9000e40d018 of 8 bytes by task 7604 on cpu 1:
 add_echo_byte drivers/tty/n_tty.c:841 [inline]
 echo_char drivers/tty/n_tty.c:940 [inline]
 n_tty_receive_char_fast drivers/tty/n_tty.c:1444 [inline]
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1605 [inline]
 __receive_buf drivers/tty/n_tty.c:1640 [inline]
 n_tty_receive_buf_common+0x1285/0x1e20 drivers/tty/n_tty.c:1739
 n_tty_receive_buf+0x2b/0x40 drivers/tty/n_tty.c:1768
 tiocsti drivers/tty/tty_io.c:2316 [inline]
 tty_ioctl+0x96f/0x1130 drivers/tty/tty_io.c:2717
 vfs_ioctl fs/ioctl.c:48 [inline]
 __do_sys_ioctl fs/ioctl.c:753 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:739
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:739
 do_syscall_64+0x34/0x50 arch/x86/entry/common.c:46
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffc9000e40d018 of 8 bytes by task 22 on cpu 0:
 echo_char drivers/tty/n_tty.c:840 [inline]
 n_tty_receive_char_fast drivers/tty/n_tty.c:1444 [inline]
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1605 [inline]
 __receive_buf drivers/tty/n_tty.c:1640 [inline]
 n_tty_receive_buf_common+0x1363/0x1e20 drivers/tty/n_tty.c:1739
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1774
 tty_ldisc_receive_buf+0x61/0xf0 drivers/tty/tty_buffer.c:461
 tty_port_default_receive_buf+0x54/0x80 drivers/tty/tty_port.c:38
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1c5/0x2c0 drivers/tty/tty_buffer.c:533
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2275
 worker_thread+0x636/0xae0 kernel/workqueue.c:2421
 kthread+0x1d0/0x1f0 kernel/kthread.c:292
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 22 Comm: kworker/u4:1 Not tainted 5.12.0-rc7-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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