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BUG: KCSAN: data-race in __d_lookup / __put_mountpoint

write to 0xffff88811dd5cc00 of 4 bytes by task 25579 on cpu 0:
 __put_mountpoint+0x93/0x130 fs/namespace.c:776
 put_mountpoint fs/namespace.c:787 [inline]
 attach_recursive_mnt+0x1027/0x1210 fs/namespace.c:2164
 graft_tree fs/namespace.c:2237 [inline]
 do_add_mount fs/namespace.c:2783 [inline]
 do_new_mount_fc fs/namespace.c:2822 [inline]
 do_new_mount fs/namespace.c:2876 [inline]
 do_mount+0x1aa8/0x1bd0 fs/namespace.c:3199
 __do_sys_mount fs/namespace.c:3409 [inline]
 __se_sys_mount+0xd3/0x100 fs/namespace.c:3386
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3386
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88811dd5cc00 of 4 bytes by task 8804 on cpu 1:
 d_same_name fs/dcache.c:2194 [inline]
 __d_lookup+0x16f/0x3b0 fs/dcache.c:2399
 lookup_fast+0x67/0x370 fs/namei.c:1502
 walk_component+0x5c/0x350 fs/namei.c:1856
 lookup_last fs/namei.c:2309 [inline]
 path_lookupat+0x12d/0x560 fs/namei.c:2333
 filename_lookup+0xf2/0x380 fs/namei.c:2366
 user_path_at_empty+0x3b/0x50 fs/namei.c:2646
 user_path_at include/linux/namei.h:59 [inline]
 ksys_umount+0xc2/0xa80 fs/namespace.c:1733
 __do_sys_umount fs/namespace.c:1759 [inline]
 __se_sys_umount fs/namespace.c:1757 [inline]
 __x64_sys_umount+0x2c/0x40 fs/namespace.c:1757
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:359
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 8804 Comm: syz-executor.1 Not tainted 5.8.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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