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BUG: KCSAN: data-race in pty_write_room / start_tty

write to 0xffff888108de6dbc of 1 bytes by task 6419 on cpu 0:
 __start_tty drivers/tty/tty_io.c:787 [inline]
 start_tty+0x81/0x170 drivers/tty/tty_io.c:809
 n_tty_set_termios+0x6ea/0x820 drivers/tty/n_tty.c:1836
 tty_set_termios+0x84e/0x8a0 drivers/tty/tty_ioctl.c:370
 set_termios+0x251/0x280 drivers/tty/tty_ioctl.c:517
 tty_mode_ioctl+0x405/0x470
 n_tty_ioctl_helper+0x83/0x340 drivers/tty/tty_ioctl.c:980
 n_tty_ioctl+0xfc/0x1f0 drivers/tty/n_tty.c:2483
 tty_ioctl+0x6e9/0x940 drivers/tty/tty_io.c:2780
 vfs_ioctl fs/ioctl.c:51 [inline]
 __do_sys_ioctl fs/ioctl.c:870 [inline]
 __se_sys_ioctl+0xcb/0x140 fs/ioctl.c:856
 __x64_sys_ioctl+0x3f/0x50 fs/ioctl.c:856
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

read to 0xffff888108de6dbc of 1 bytes by task 6406 on cpu 1:
 pty_write_room+0x18/0x80 drivers/tty/pty.c:131
 tty_write_room+0x34/0x50 drivers/tty/tty_ioctl.c:80
 n_tty_poll+0x461/0x490 drivers/tty/n_tty.c:2441
 tty_poll+0x77/0xf0 drivers/tty/tty_io.c:2205
 vfs_poll include/linux/poll.h:88 [inline]
 ep_item_poll fs/eventpoll.c:853 [inline]
 ep_insert+0x835/0xbb0 fs/eventpoll.c:1522
 do_epoll_ctl+0x59c/0x890 fs/eventpoll.c:2163
 __do_sys_epoll_ctl fs/eventpoll.c:2214 [inline]
 __se_sys_epoll_ctl fs/eventpoll.c:2205 [inline]
 __x64_sys_epoll_ctl+0xc2/0xf0 fs/eventpoll.c:2205
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x2b/0x70 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x63/0xcd

value changed: 0x01 -> 0x00

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6406 Comm: syz-executor.1 Tainted: G        W          6.1.0-rc7-syzkaller-00102-g04aa64375f48-dirty #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
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