backtrace of fprintd while it's hung

#0  0x00007fbcdde84287 in statfs64 () from target:/lib64/libc.so.6
#1  0x00007fbcdde84309 in statvfs64 () from target:/lib64/libc.so.6
#2  0x00005627d4a130f7 in get_mount_flags (
    path=path@entry=0x5627d5f54710 "/media/synology/backup", 
    flags=flags@entry=0x7ffc8e276778) at ../src/basic/mount-util.c:314
#3  0x00005627d4a1577a in bind_remount_recursive_with_mountinfo (ro=true, 
    proc_self_mountinfo=<optimized out>, blacklist=<optimized out>, 
    prefix=<optimized out>) at ../src/basic/mount-util.c:489
#4  make_read_only (proc_self_mountinfo=<optimized out>, 
    blacklist=<optimized out>, m=<optimized out>)
    at ../src/core/namespace.c:803
#5  setup_namespace.constprop.59 (root_directory=<optimized out>, 
    root_image=<optimized out>, ns_info=<optimized out>, 
    read_write_paths=<optimized out>, read_only_paths=<optimized out>, 
    inaccessible_paths=<optimized out>, bind_mounts=<optimized out>, 
    n_bind_mounts=<optimized out>, tmp_dir=<optimized out>, 
    var_tmp_dir=<optimized out>, protect_home=<optimized out>, 
    protect_system=<optimized out>, mount_flags=<optimized out>, 
    dissect_image_flags=<optimized out>) at ../src/core/namespace.c:1115
#6  0x00005627d4a17654 in apply_mount_namespace.lto_priv.263 (
    u=0x5627d610b5b0, command=0x5627d606cc40, context=0x5627d610b9e0, 
    params=<optimized out>, runtime=<optimized out>)
    at ../src/core/execute.c:2002
#7  0x00005627d4a851f4 in exec_child.constprop.47 (unit=0x5627d610b5b0, 
    command=0x5627d606cc40, context=<optimized out>, params=0x7ffc8e277180, 
    runtime=<optimized out>, dcreds=<optimized out>, argv=<optimized out>, 
    socket_fd=<optimized out>, named_iofds=<optimized out>, 
    fds=<optimized out>, n_storage_fds=<optimized out>, 
    n_socket_fds=<optimized out>, files_env=<optimized out>, 
    user_lookup_fd=<optimized out>, exit_status=<optimized out>, 
    error_message=<optimized out>) at ../src/core/execute.c:2626
#8  0x00005627d4ab5060 in exec_spawn (unit=<optimized out>, 
    command=0x5627d606cc40, context=0x5627d610b9e0, params=<optimized out>, 
    runtime=0x0, dcreds=0x5627d610bd28, ret=0x7ffc8e277170)
    at ../src/core/execute.c:2983
#9  0x00005627d4a5801f in service_spawn (s=s@entry=0x5627d610b5b0, 
    c=<optimized out>, timeout=<optimized out>, 
    flags=flags@entry=(EXEC_APPLY_PERMISSIONS | EXEC_APPLY_CHROOT | EXEC_APPLY_TTY_STDIN | EXEC_PASS_FDS | EXEC_SET_WATCHDOG), _pid=_pid@entry=0x7ffc8e2772c4)
    at ../src/core/service.c:1375
#10 0x00005627d4a60c6c in service_enter_start (s=s@entry=0x5627d610b5b0)
    at ../src/core/service.c:1817
#11 0x00005627d4a61098 in service_enter_start_pre (s=0x5627d610b5b0)
    at ../src/core/service.c:1886
#12 service_start.lto_priv.375 (u=0x5627d610b5b0) at ../src/core/service.c:2099
#13 0x00005627d4a9ad3c in unit_start (u=0x5627d610b5b0)
    at ../src/core/unit.c:1647
#14 job_perform_on_unit.lto_priv.870 (j=0x7ffc8e2773a0)
    at ../src/core/job.c:535
#15 0x00005627d4ab83d8 in job_run_and_invalidate (j=<optimized out>)
    at ../src/core/job.c:600
#16 manager_dispatch_run_queue.lto_priv.881 (source=<optimized out>, 
    userdata=<optimized out>, userdata=<optimized out>)
    at ../src/core/manager.c:1621
#17 0x00007fbcdda6f4ba in source_dispatch (s=s@entry=0x5627d5f75930)
    at ../src/libsystemd/sd-event/sd-event.c:2312
#18 0x00007fbcdda6f7da in sd_event_dispatch (e=e@entry=0x5627d5f75c10)
    at ../src/libsystemd/sd-event/sd-event.c:2631
#19 0x00007fbcdda6f957 in sd_event_run (e=0x5627d5f75c10, 
    timeout=18446744073709551615) at ../src/libsystemd/sd-event/sd-event.c:2690
#20 0x00005627d4abc5a2 in manager_loop (m=0x5627d5f77520)
    at ../src/core/manager.c:2291
#21 0x00005627d4a1ece1 in main (argc=5, argv=<optimized out>)
    at ../src/core/main.c:1937