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BUG: KCSAN: data-race in n_tty_poll / pty_unix98_ioctl

write to 0xffff88812e19d1dc of 2 bytes by task 10870 on cpu 1:
 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 0xffff88812e19d1dc of 2 bytes by task 10882 on cpu 0:
 n_tty_poll+0x363/0x4a0 drivers/tty/n_tty.c:2437
 tty_poll+0xa7/0x140 drivers/tty/tty_io.c:2231
 vfs_poll include/linux/poll.h:90 [inline]
 do_pollfd fs/select.c:869 [inline]
 do_poll fs/select.c:917 [inline]
 do_sys_poll+0x622/0xc40 fs/select.c:1011
 __do_sys_poll fs/select.c:1076 [inline]
 __se_sys_poll+0xce/0x1e0 fs/select.c:1064
 __x64_sys_poll+0x3f/0x50 fs/select.c:1064
 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: 10882 Comm: syz-executor.1 Not tainted 5.13.0-rc1-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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