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BUG: KCSAN: data-race in n_tty_ioctl_helper / uart_write

write to 0xffff88812f1e01cc of 1 bytes by task 24193 on cpu 1:
 n_tty_ioctl_helper+0x106/0x4c0 drivers/tty/tty_ioctl.c:851
 n_tty_ioctl+0x102/0x200 drivers/tty/n_tty.c:2487
 tty_ioctl+0xa8f/0x1120 drivers/tty/tty_io.c:2814
 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 0xffff88812f1e01cc of 1 bytes by task 24181 on cpu 0:
 uart_tx_stopped include/linux/serial_core.h:431 [inline]
 __uart_start drivers/tty/serial/serial_core.c:126 [inline]
 uart_write+0x35b/0x5c0 drivers/tty/serial/serial_core.c:614
 do_output_char+0x47f/0x4f0 drivers/tty/n_tty.c:444
 process_output drivers/tty/n_tty.c:511 [inline]
 n_tty_write+0x7f8/0xaf0 drivers/tty/n_tty.c:2366
 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_iter_write+0x4c/0x70 fs/read_write.c:907
 iter_file_splice_write+0x40a/0x750 fs/splice.c:689
 do_splice_from fs/splice.c:767 [inline]
 direct_splice_actor+0x80/0xa0 fs/splice.c:936
 splice_direct_to_actor+0x345/0x650 fs/splice.c:891
 do_splice_direct+0xf5/0x170 fs/splice.c:979
 do_sendfile+0x773/0xda0 fs/read_write.c:1260
 __do_sys_sendfile64 fs/read_write.c:1325 [inline]
 __se_sys_sendfile64 fs/read_write.c:1311 [inline]
 __x64_sys_sendfile64+0xf2/0x130 fs/read_write.c:1311
 do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x00 -> 0x03

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 24181 Comm: syz-executor.0 Not tainted 5.13.0-rc5-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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