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BUG: KCSAN: data-race in __process_echoes / n_tty_write

write to 0xffffc900012b7278 of 8 bytes by task 9441 on cpu 0:
 __process_echoes+0x607/0x660 drivers/tty/n_tty.c:751
 flush_echoes drivers/tty/n_tty.c:812 [inline]
 __receive_buf drivers/tty/n_tty.c:1551 [inline]
 n_tty_receive_buf_common+0x4c38/0x4e70 drivers/tty/n_tty.c:1645
 n_tty_receive_buf+0x2b/0x40 drivers/tty/n_tty.c:1674
 tiocsti drivers/tty/tty_io.c:2293 [inline]
 tty_ioctl+0x978/0x1130 drivers/tty/tty_io.c:2692
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:874 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:860
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:860
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffc900012b7278 of 8 bytes by task 9432 on cpu 1:
 process_echoes drivers/tty/n_tty.c:789 [inline]
 n_tty_write+0x148/0xaf0 drivers/tty/n_tty.c:2234
 do_tty_write drivers/tty/tty_io.c:1024 [inline]
 file_tty_write+0x409/0x660 drivers/tty/tty_io.c:1095
 tty_write+0x24/0x30 drivers/tty/tty_io.c:1116
 call_write_iter include/linux/fs.h:2074 [inline]
 new_sync_write fs/read_write.c:503 [inline]
 vfs_write+0x7f5/0x950 fs/read_write.c:590
 ksys_write+0xd9/0x190 fs/read_write.c:643
 __do_sys_write fs/read_write.c:655 [inline]
 __se_sys_write fs/read_write.c:652 [inline]
 __x64_sys_write+0x3e/0x50 fs/read_write.c:652
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x0000000000000000 -> 0x0000000000000003

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9432 Comm: syz-executor.4 Tainted: G        W         5.17.0-syzkaller-01442-gb47d5a4f6b8d-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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