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BUG: KCSAN: data-race in tty_flip_buffer_push / tty_insert_flip_string_fixed_flag

write to 0xffff88813972fc08 of 4 bytes by task 24807 on cpu 1:
 tty_insert_flip_string_fixed_flag+0x15c/0x1c0 drivers/tty/tty_buffer.c:329
 tty_insert_flip_string include/linux/tty_flip.h:42 [inline]
 pty_write+0xbc/0x120 drivers/tty/pty.c:122
 tty_send_xchar+0x1ce/0x290 drivers/tty/tty_io.c:1182
 n_tty_ioctl_helper+0x27f/0x4c0
 n_tty_ioctl+0x102/0x200 drivers/tty/n_tty.c:2421
 tty_ioctl+0xaf9/0x11a0 drivers/tty/tty_io.c:2814
 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+0x44/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813972fc08 of 4 bytes by task 520 on cpu 0:
 tty_schedule_flip drivers/tty/tty_buffer.c:420 [inline]
 tty_flip_buffer_push+0x28/0x60 drivers/tty/tty_buffer.c:566
 pty_write+0xfe/0x120 drivers/tty/pty.c:126
 tty_put_char+0x7f/0xa0 drivers/tty/tty_io.c:3174
 __process_echoes+0x3bd/0x660 drivers/tty/n_tty.c:726
 flush_echoes drivers/tty/n_tty.c:826 [inline]
 __receive_buf drivers/tty/n_tty.c:1579 [inline]
 n_tty_receive_buf_common+0x4c38/0x4e70 drivers/tty/n_tty.c:1674
 n_tty_receive_buf2+0x2e/0x40 drivers/tty/n_tty.c:1709
 tty_ldisc_receive_buf+0x5b/0xf0 drivers/tty/tty_buffer.c:471
 tty_port_default_receive_buf+0x54/0x80 drivers/tty/tty_port.c:39
 receive_buf drivers/tty/tty_buffer.c:491 [inline]
 flush_to_ldisc+0x1c5/0x2c0 drivers/tty/tty_buffer.c:543
 process_one_work+0x402/0x910 kernel/workqueue.c:2297
 worker_thread+0x636/0xae0 kernel/workqueue.c:2444
 kthread+0x262/0x280 kernel/kthread.c:319
 ret_from_fork+0x1f/0x30

value changed: 0x00000121 -> 0x00000122

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 520 Comm: kworker/u4:5 Tainted: G        W         5.15.0-rc6-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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