Hard lockup with 2.6.24.7-rt26 on x86

[m.luescher@vtxmail.ch]

Dear Linux rt users

On two different desktop PC (both Core 2 Duo) and on a notebook (Core Duo) the kernel (2.6.24.7-rt26, CONFIG_X86_32) locks up hard under a certain rt load. The failure can be reproduced reliably with the following command:

sudo ./cyclictest -p99 -t10 -n -i250

During the test I made some additional observations:
- The lockup is really hard: there is no output on a netconsole and even the magic sysrequest keys to reboot the system (Alt+SysRQ+b) do not work anymore.
- with a lower realtime priority the system seems to run at least more stable (e.g. sudo ./cyclictest -p95 -t10 -n -i250)
- with the vanilla 2.6.26.6-rt11 kernel I observed probably the same crash
- the problem also appears on Ubuntu Hardy Heron with the latest official kernel (2.6.24.23.25 with rt21 patch)
- the problem did not appear on Ubuntu Hardy Heron with older rt-kernel versions (also 2.6.24 but based on rt3 patch)

So at the moment I am really clueless and I would be happy for any hint on how I can further debug this crash.

Best regards
Matthias