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BUG: KCSAN: data-race in __process_echoes / n_tty_write

write to 0xffffc9000ecfb278 of 8 bytes by task 26930 on cpu 1:
 __process_echoes+0x607/0x660 drivers/tty/n_tty.c:765
 flush_echoes drivers/tty/n_tty.c:826 [inline]
 __receive_buf drivers/tty/n_tty.c:1645 [inline]
 n_tty_receive_buf_common+0x1ba4/0x1e20 drivers/tty/n_tty.c:1740
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1775
 tty_ldisc_receive_buf+0x5b/0xf0 drivers/tty/tty_buffer.c:461
 paste_selection+0x250/0x340 drivers/tty/vt/selection.c:408
 tioclinux+0xe5/0x450 drivers/tty/vt/vt.c:3164
 vt_ioctl+0x26e/0x1ed0 drivers/tty/vt/vt_ioctl.c:723
 tty_ioctl+0xa24/0x1120 drivers/tty/tty_io.c:2805
 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_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffffc9000ecfb278 of 8 bytes by task 26931 on cpu 0:
 process_echoes drivers/tty/n_tty.c:803 [inline]
 n_tty_write+0x147/0xaf0 drivers/tty/n_tty.c:2340
 do_tty_write drivers/tty/tty_io.c:1043 [inline]
 file_tty_write+0x409/0x660 drivers/tty/tty_io.c:1133
 tty_write+0x24/0x30 drivers/tty/tty_io.c:1140
 call_write_iter include/linux/fs.h:2114 [inline]
 do_iter_readv_writev+0x2cb/0x360 fs/read_write.c:740
 do_iter_write+0x112/0x4c0 fs/read_write.c:866
 vfs_writev fs/read_write.c:939 [inline]
 do_writev+0x1e0/0x400 fs/read_write.c:982
 __do_sys_writev fs/read_write.c:1055 [inline]
 __se_sys_writev fs/read_write.c:1052 [inline]
 __x64_sys_writev+0x41/0x50 fs/read_write.c:1052
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 26931 Comm: syz-executor.2 Tainted: G        W         5.12.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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