[Help] sysfs is already mounted or busy.

我用的fedora 14，昨天想升级到fedora 15，升级bootloader前后电脑挂了，黑屏加一个鼠标忙碌的图片。
重启，回fedora 14，顺利启动，进入系统。
不过有报错。

Mounting local filesystems: mount: sysfs already mounted or /sys busy
mount: according to mtab, /sys is already mounted on /sys
  [FAILED]

这个是怎么回事呢？
[root@pek-yiwei ~]# ls /sys
block bus class dev devices firmware fs kernel module power
[root@pek-yiwei ~]# umount /sys
umount: /sys: device is busy.
  (In some cases useful info about processes that use
  the device is found by lsof(8) or fuser(1))

sys目录似乎已经mount了，为啥还要mount呢？
谢谢了。

------解决方案--------------------
/etc/fstab里的内容，是不是有重样的了？