I just released Linux-2.1.116. I've tested it fairly extensively on my SMP
box, both with little memory and much, and I cannot make it lock up any
more.

Special thanks to Dean Gaudet who helped me set up a apache configuration
that finally made me able to repeat the lockup, and made me able to debug
the thing.

Most of the 2.1.116 patches are "just" alpha and m68k updates, and can be
ignored by most people. The bugfixes are, roughly:

 - fixed serious low-memory situation problem, where a critical resource
   allocation problem could result in nasy behaviour. Notably, doing TCP
   under low memory could result in TCP trying to allocate memory in a
   tight loop and locking out kswapd completely so that the situation
   would never be rectified. In short, the machine hung.
   This problem has been there forever, the only reason it doesn't show up
   under 2.0.x seems to be because under 2.0.x the TCP allocation was
   always for a single page, for which this situation never arises. Under
   2.1.x the slab code forced multi-page allocations.
   If you've seen lockups with 2.1.x, this may be the cause. This was what
   held up 2.1.116 for so long.
 - various minor driver updates. Networking, radio, bttv.
 - NFS over TCP still doesn't work, but at least it fails due to new
   reasons.

Alan, try your squid thing under 2.1.116. I suspect it will hold up now,

                Linus