It seems that roughly half the time "shutdown -r now" will hang on
"Syncing disks..." with the sync operation complete.  The kernel
debugger shows that it's trapped within the bowels of vfs_umountall.
I have seen this behavior for months now, probably when the earliest
versions of vfs_umountall were beginning to take shape.

I have never seen the system hang if you use "reboot" instead of
"shutdown -r now".

This happens to me a two fairly different PCs.

Do other people see both of these behaviors?

Ken