Using read-only NULLFS leads to panic. gdb output included, easy to reproduce.

[Pawel Malachowski]

On Wed, Mar 03, 2004 at 11:25:36AM +1100, Tim Robbins wrote:

> There are known bugs in nullfs in all 4.x releases to date, and in 5.0.
> If I have time, I may MFC the fixes some time before 4.10 is released.

Sounds interesting.

> Can you reproduce these problems on 5.1 or 5.2?

Procedure:
mount_null -o ro /usr/ports /mnt/1
find /usr/ports -type f -perm -u+s &
find /mnt/1 -type f -perm -u+s &
...
<about 30-40 processess of find(1) was enough>


I was not able to reproduce panic on 5.1-RELEASE system.

However, I've reproduced second problem described in this PR,
which happened to me on 4.9-STABLE once.
*Every* time (I've tried four times) 5.1-RELEASE becomes unusable
and non-rebootable, all find(1) processes stuck into 'ufs' state,
CPU is 100% idle, no disk activity, other processess, when trying
to read from filesystem, become frozen. I'm able to switch beetween
consoles and to observe top(1) (started earlier). Ctrl+Alt+Del
does not work.

One mount_null is sufficient, but more than one null mount seems
(or it was an accident) to decrease the time we have to wait.


-- 
Paweł Małachowski