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BUG: KCSAN: data-race in __put_mountpoint / dput

write to 0xffff88812b8029c0 of 4 bytes by task 29080 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:2784 [inline]
 do_new_mount_fc fs/namespace.c:2823 [inline]
 do_new_mount fs/namespace.c:2877 [inline]
 do_mount+0x1ad5/0x1c00 fs/namespace.c:3200
 __do_sys_mount fs/namespace.c:3410 [inline]
 __se_sys_mount+0xd3/0x100 fs/namespace.c:3387
 __x64_sys_mount+0x63/0x70 fs/namespace.c:3387
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

read to 0xffff88812b8029c0 of 4 bytes by task 9053 on cpu 1:
 fast_dput fs/dcache.c:746 [inline]
 dput+0x2f/0x420 fs/dcache.c:865
 handle_mounts fs/namei.c:1389 [inline]
 step_into+0x448/0xe70 fs/namei.c:1690
 open_last_lookups fs/namei.c:3192 [inline]
 path_openat+0xfca/0x20e0 fs/namei.c:3357
 do_filp_open+0x198/0x1d0 fs/namei.c:3389
 do_sys_openat2+0x33c/0x4a0 fs/open.c:1179
 do_sys_open fs/open.c:1195 [inline]
 ksys_open include/linux/syscalls.h:1388 [inline]
 __do_sys_open fs/open.c:1201 [inline]
 __se_sys_open fs/open.c:1199 [inline]
 __x64_sys_open+0xe2/0x110 fs/open.c:1199
 do_syscall_64+0x51/0xb0 arch/x86/entry/common.c:384
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 9053 Comm: syz-executor.3 Not tainted 5.8.0-rc7-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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