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BUG: KCSAN: data-race in n_tty_read / pty_stop

write to 0xffff8881297175dc of 2 bytes by task 22217 on cpu 0:
 pty_stop+0x9c/0xf0 drivers/tty/pty.c:365
 __stop_tty drivers/tty/tty_io.c:790 [inline]
 stop_tty drivers/tty/tty_io.c:798 [inline]
 tty_send_xchar+0x1e2/0x290 drivers/tty/tty_io.c:1192
 n_tty_ioctl_helper+0x2a3/0x4c0 drivers/tty/tty_ioctl.c:870
 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_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff8881297175dc of 2 bytes by task 22198 on cpu 1:
 n_tty_read+0x70c/0x10c0 drivers/tty/n_tty.c:2203
 iterate_tty_read drivers/tty/tty_io.c:873 [inline]
 tty_read+0x18c/0x410 drivers/tty/tty_io.c:950
 call_read_iter include/linux/fs.h:2108 [inline]
 new_sync_read fs/read_write.c:415 [inline]
 vfs_read+0x565/0x5d0 fs/read_write.c:496
 ksys_read+0xce/0x180 fs/read_write.c:634
 __do_sys_read fs/read_write.c:644 [inline]
 __se_sys_read fs/read_write.c:642 [inline]
 __x64_sys_read+0x3e/0x50 fs/read_write.c:642
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x3d/0x90 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x0008 -> 0x0004

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 22198 Comm: syz-executor.2 Tainted: G        W         5.13.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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