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BUG: KCSAN: data-race in d_set_mounted / dput

write to 0xffff88808e4ef000 of 4 bytes by task 2869 on cpu 0:
 d_set_mounted+0xfc/0x190 fs/dcache.c:1459
 get_mountpoint+0x1be/0x300 fs/namespace.c:740
 attach_recursive_mnt+0x9d/0x1210 fs/namespace.c:2114
 graft_tree fs/namespace.c:2237 [inline]
 do_loopback fs/namespace.c:2365 [inline]
 do_mount+0x121d/0x1c00 fs/namespace.c:3194
 __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 0xffff88808e4ef000 of 4 bytes by task 2862 on cpu 1:
 fast_dput fs/dcache.c:746 [inline]
 dput+0x2f/0x420 fs/dcache.c:865
 cleanup_mnt+0x265/0x2e0 fs/namespace.c:1117
 __cleanup_mnt+0x15/0x20 fs/namespace.c:1125
 task_work_run+0x8e/0x110 kernel/task_work.c:135
 tracehook_notify_resume include/linux/tracehook.h:188 [inline]
 exit_to_usermode_loop arch/x86/entry/common.c:239 [inline]
 __prepare_exit_to_usermode+0x1c6/0x230 arch/x86/entry/common.c:269
 __syscall_return_slowpath+0x47/0x60 arch/x86/entry/common.c:352
 do_syscall_64+0x5d/0xb0 arch/x86/entry/common.c:393
 entry_SYSCALL_64_after_hwframe+0x44/0xa9

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 2862 Comm: syz-executor.5 Not tainted 5.8.0-rc6-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
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