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BUG: KCSAN: data-race in __lookup_mnt / umount_tree

write to 0xffff888115bb4010 of 8 bytes by task 6801 on cpu 0:
 unhash_mnt fs/namespace.c:891 [inline]
 umount_mnt fs/namespace.c:906 [inline]
 umount_tree+0x625/0x910 fs/namespace.c:1673
 path_umount+0x991/0xa10 fs/namespace.c:1896
 ksys_umount fs/namespace.c:1919 [inline]
 __do_sys_umount fs/namespace.c:1924 [inline]
 __se_sys_umount fs/namespace.c:1922 [inline]
 __x64_sys_umount+0xb9/0xe0 fs/namespace.c:1922
 do_syscall_64+0xd3/0x1d0
 entry_SYSCALL_64_after_hwframe+0x72/0x7a

read to 0xffff888115bb4010 of 8 bytes by task 6803 on cpu 1:
 __lookup_mnt+0x83/0xf0 fs/namespace.c:701
 __follow_mount_rcu fs/namei.c:1506 [inline]
 handle_mounts fs/namei.c:1536 [inline]
 step_into+0x434/0x810 fs/namei.c:1842
 walk_component+0x169/0x230 fs/namei.c:2010
 lookup_last fs/namei.c:2461 [inline]
 path_lookupat+0x10a/0x2b0 fs/namei.c:2485
 filename_lookup+0x127/0x300 fs/namei.c:2514
 user_path_at_empty+0x42/0x120 fs/namei.c:2921
 user_path_at include/linux/namei.h:57 [inline]
 ksys_umount fs/namespace.c:1916 [inline]
 __do_sys_umount fs/namespace.c:1924 [inline]
 __se_sys_umount fs/namespace.c:1922 [inline]
 __x64_sys_umount+0x88/0xe0 fs/namespace.c:1922
 do_syscall_64+0xd3/0x1d0
 entry_SYSCALL_64_after_hwframe+0x72/0x7a

value changed: 0xffff888104009980 -> 0xffff888115bb4000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 6803 Comm: syz-executor.4 Tainted: G        W          6.9.0-rc3-syzkaller-00011-g20cb38a7af88 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 03/27/2024
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