I have an IBM xSeries with 2 CPUs à 2,4GHz with Hyper-Threading (makes 4 virtual CPUs à 2,4GHz) and 2 GB RAM running Ubuntu Feisty (7.04) as host system.
Running three VMs via commands like "nohup VBoxVRDP -startvm sles10-node1 > /home/virtualbox/Machines/sles10-node1/Logs/nohup.log 2>&1 &" works without problems. Adding and starting a fourth node leads to the problem, that the kernel of the host completely freezes when trying to poweroff (VBoxManage controlvm sles10-node1 poweroff) or kill (kill -9 <pid>) one of the four processes for the four running VMs. On the console the following message appears: "BUG: soft lock detected for CPU#3".
Passing the parameter "nostflock" to the kernel via the menu.lst in /boot/grub/ was tried, but with the same result! :/
Disabling Hyper-Threading in the BIOS didn't improve the issue either.
A memtest86+ will run this night, but we don't really think the RAM is broken or something, but you never know...
If the test does not indicate any errors tomorrow morning, the issue still remains unsolved... :(
The strange thing is that this issue only occurs when running 4 VMs (with 3 all works fine) and only when trying to poweroff or kill one of them. Until then, they run happily!
An extract of /var/log/messages is attached...