deadlocks with SMP and Pentium 3

Hello,

I have an Asus CUVX4-D mainboard with an VIA Apollo Pro 133A chipset and 
two Intel Pentium III 733 MHz processors. It runs fine with FreeBSD 4.11 
and SMP enabled, but something happened before 5.0-RELEASE that it don't 
work anymore now; neither with 5.4-RELEASE, 6.0-RC1 nor 7-CURRENT. In 
case I have many concurrent processes the system just locks up after a 
few minutes.

I disabled ACPI, compiled a kernel with WITNESS, set all debug mpsafe 
tunables to zero and many other things, but nothing helped to find a 
reason for this behaviour. Right now I noticed that top(1) shows a huge 
bunch of processes with the state *Giant shortly before the machine 
freezed. Therefore I think it's related to the giant lock. ;-)

Currently I prepare to do a binary search in HEAD between march 2000 and 
januar 2003, but I would be glad if somebody still has a hint for me to 
prevent me from doing this. Is there still a way to gather debug 
information?

Regards
Björn