我安装了 Ubuntu 18.04 LTS（这是全新安装，我没有从旧版本升级）。从那时起，每当我关闭它时，它总是挂起，有时如下图所示：
其他时候，它会挂起，如下所示：
然后，我必须按下电源按钮强制其关机。
我在 Google 上搜索了一下，找到了这个建议：更改GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"为GRUB_CMDLINE_LINUX_DEFAULT=""然后更新 grub。
我按照它说的做了，但没有用。
一些附加信息：
$ uname -a
Linux My-FRIEND 4.15.0-20-generic #21-Ubuntu SMP Tue Apr 24 06:16:15 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
我运行时发现一个可疑的部分dmesg：
[ 31.329263] IPv6: ADDRCONF(NETDEV_UP): enp3s0: link is not ready
[ 31.414525] r8169 0000:03:00.0 enp3s0: link down
[ 31.414609] IPv6: ADDRCONF(NETDEV_UP): enp3s0: link is not ready
[ 31.484384] IPv6: ADDRCONF(NETDEV_UP): wlp2s0: link is not ready
[ 37.467771] PKCS#7 signature not signed with a trusted key
[ 37.467779] vboxdrv: loading out-of-tree module taints kernel.
[ 37.467971] vboxdrv: module verification failed: signature and/or required key missing - tainting kernel
[ 37.474593] vboxdrv: Found 4 processor cores
[ 37.492246] vboxdrv: TSC mode is Invariant, tentative frequency 2194900161 Hz
[ 37.492247] vboxdrv: Successfully loaded version 5.2.10_Ubuntu (interface 0x00290001)
[ 37.501075] PKCS#7 signature not signed with a trusted key
[ 37.501461] VBoxNetFlt: Successfully started.
[ 37.506348] PKCS#7 signature not signed with a trusted key
[ 37.506631] VBoxNetAdp: Successfully started.
[ 37.511121] PKCS#7 signature not signed with a trusted key
[ 37.511416] VBoxPciLinuxInit
[ 37.513435] vboxpci: IOMMU not found (not registered)
这与内核或我的 WiFi 卡有关吗？