umount hangs with initramfs as root

[Chuck Meade]

Hi Rob,

Thanks for the info.

> > Busybox umount hangs when I try to reboot, while running on a root
> > fs which is an initramfs.  Reboot is configured on my system to
> > "umount -a -r", and the umount of the initramfs root is hanging.
> 
> You're using a kernel before 2.6.13, aren't you?

Yes I am working on a few platforms at once, and this particular one is
still at 2.6.11.6.  The Busybox version running on it is 1.0.0.
I know they are not very recent, but I was trying to figure out
if the problem with umounting an initramfs is known/fixed, or known
to not work regardless of the version, etc.

> I forgot: the patch to prevent initramfs from being umounted went in around 
> then.  Before that you could hang your system hard by umounting initramfs.  
> (The kernel would go into an endless loop since the mount list is a circular 
> doubly linked list with initramfs as the start/end point it searches for.)
> 
> Hmmm...  Didn't test umount -a with an older kernel.

This is what I mean -- am I correct in interpreting what you wrote above,
that an initramfs simply cannot be umounted?  

I have used initrd's in the past, and am testing out using initramfs, as
my embedded rootfs  (not just a transitional rootfs).  It works great until
I try to reboot, and the umount hangs.  Especially for a small embedded
target with limited resources, having the initramfs as the "real" rootfs
has advantages.  This umount problem is not one of them though :)

I don't know if this inability to umount the initramfs is a "feature" or
a bug.  Since you say that a patch went in to prevent it, maybe it is
desired behavior.  But it does not make it easy to umount for rebooting.

I see that you wrote the Linux Documentation/ file on initramfs.  So 
based on your knowledge of this topic, what do you think is the appropriate
next step here?  I am thinking that possibilities include:
1. changing my busybox reboot sequence to not "umount -a -r", just umount
   the mounts *under* /
2. possibly changing the reboot sequence to switch_root to some other
   directory first, then umounting initramfs and rebooting
3. any other better ideas?

What would you suggest?

> I tweaked umount a bit, but it's possible this specific issue didn't get 
> addressed.
> 
> What versions of busybox and the kernel are you using?  I'll see if I can 
> reproduce it.  Is this a recent problem, or has busybox been doing it for a 
> while?

My versions are not recent so they do reflect any of your recent mount work.

Thanks very much for your thoughts on this,
Chuck