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BUG: KCSAN: data-race in isig / pty_unix98_ioctl

write to 0xffff8881302975dc of 2 bytes by task 13490 on cpu 0:
 pty_set_pktmode drivers/tty/pty.c:196 [inline]
 pty_unix98_ioctl+0x266/0x350 drivers/tty/pty.c:676
 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 0xffff8881302975dc of 2 bytes by task 9298 on cpu 1:
 n_tty_packet_mode_flush drivers/tty/n_tty.c:345 [inline]
 isig+0x1b1/0x280 drivers/tty/n_tty.c:1146
 n_tty_receive_signal_char drivers/tty/n_tty.c:1240 [inline]
 n_tty_receive_char_special+0x4bb/0x3ac0 drivers/tty/n_tty.c:1292
 n_tty_receive_buf_fast drivers/tty/n_tty.c:1607 [inline]
 __receive_buf drivers/tty/n_tty.c:1641 [inline]
 n_tty_receive_buf_common+0xe0c/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
 tty_port_default_receive_buf+0x54/0x80 drivers/tty/tty_port.c:39
 receive_buf drivers/tty/tty_buffer.c:481 [inline]
 flush_to_ldisc+0x1c5/0x2c0 drivers/tty/tty_buffer.c:533
 process_one_work+0x3e9/0x8f0 kernel/workqueue.c:2275
 worker_thread+0x636/0xae0 kernel/workqueue.c:2421
 kthread+0x1d0/0x1f0 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9298 Comm: kworker/u4:0 Not tainted 5.13.0-rc1-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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