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BUG: KCSAN: data-race in __lookup_mnt / umount_tree

write to 0xffff888105e44990 of 8 bytes by task 9724 on cpu 1:
 unhash_mnt fs/namespace.c:839 [inline]
 umount_mnt fs/namespace.c:854 [inline]
 umount_tree+0x5ad/0x8f0 fs/namespace.c:1545
 path_umount+0x9df/0xa70 fs/namespace.c:1753
 ksys_umount fs/namespace.c:1776 [inline]
 __do_sys_umount fs/namespace.c:1781 [inline]
 __se_sys_umount fs/namespace.c:1779 [inline]
 __x64_sys_umount+0xb5/0xe0 fs/namespace.c:1779
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff888105e44990 of 8 bytes by task 9725 on cpu 0:
 __lookup_mnt+0x89/0xf0 fs/namespace.c:630
 __follow_mount_rcu fs/namei.c:1450 [inline]
 handle_mounts fs/namei.c:1486 [inline]
 step_into+0x35c/0xee0 fs/namei.c:1800
 walk_component+0x1a1/0x360 fs/namei.c:1976
 lookup_last fs/namei.c:2425 [inline]
 path_lookupat+0x12d/0x3d0 fs/namei.c:2449
 filename_lookup+0x130/0x310 fs/namei.c:2478
 user_path_at_empty+0x3e/0x110 fs/namei.c:2801
 user_path_at include/linux/namei.h:57 [inline]
 do_mount fs/namespace.c:3328 [inline]
 __do_sys_mount fs/namespace.c:3539 [inline]
 __se_sys_mount+0x217/0x2f0 fs/namespace.c:3516
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3516
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xa0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0xffff888101b84840 -> 0xffff888105e44980

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 9725 Comm: syz-executor.2 Not tainted 5.15.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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