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BUG: KCSAN: data-race in __process_echoes / n_tty_write

write to 0xffffc9000401b280 of 8 bytes by task 6652 on cpu 1:
 __process_echoes+0x688/0x6a0 drivers/tty/n_tty.c:746
 flush_echoes drivers/tty/n_tty.c:807 [inline]
 __receive_buf drivers/tty/n_tty.c:1632 [inline]
 n_tty_receive_buf_common+0x8e6/0xbc0 drivers/tty/n_tty.c:1729
 n_tty_receive_buf2+0x33/0x40 drivers/tty/n_tty.c:1775
 tty_ldisc_receive_buf+0x60/0xe0 drivers/tty/tty_buffer.c:386
 paste_selection+0x1dd/0x2e0 drivers/tty/vt/selection.c:409
 tioclinux+0x2c9/0x4a0 drivers/tty/vt/vt.c:3165
 vt_ioctl+0x84e/0x1830 drivers/tty/vt/vt_ioctl.c:762
 tty_ioctl+0x816/0xbc0 drivers/tty/tty_io.c:2803
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:871 [inline]
 __se_sys_ioctl+0xcf/0x140 fs/ioctl.c:857
 __x64_sys_ioctl+0x43/0x50 fs/ioctl.c:857
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

read to 0xffffc9000401b280 of 8 bytes by task 6704 on cpu 0:
 process_echoes drivers/tty/n_tty.c:784 [inline]
 n_tty_write+0x171/0xb60 drivers/tty/n_tty.c:2365
 iterate_tty_write drivers/tty/tty_io.c:1021 [inline]
 file_tty_write+0x382/0x680 drivers/tty/tty_io.c:1096
 tty_write+0x28/0x30 drivers/tty/tty_io.c:1117
 call_write_iter include/linux/fs.h:2085 [inline]
 new_sync_write fs/read_write.c:497 [inline]
 vfs_write+0x760/0x8d0 fs/read_write.c:590
 ksys_write+0xeb/0x1a0 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+0x42/0x50 fs/read_write.c:652
 do_syscall_x64 arch/x86/entry/common.c:52 [inline]
 do_syscall_64+0xcd/0x1d0 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

value changed: 0x00000000000037e0 -> 0x00000000000037ea

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 6704 Comm: syz-executor.0 Not tainted 6.8.0-rc3-syzkaller-00279-g4a7bbe7519b6 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/25/2024
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