Thanks for your reply. This behavior only appeared while using shared memory in form of segments. I already solved this problem by exchanging qlock, qunlock by lock (canlock). I also didn't have any problems while using other forms of memory constellations but while using shared memory segments this problem surfaced.
I need this for my vgafb implementation which makes use of shared physical and normal shared memory segments. After testing on bare metal I also found out that qlock, qunlock, rsleep, rwake ... caused kernel crashes on bare metal while usual locks work without any problems.
On 9vx I had to simulate vgafb with a implementation which provided my framebuffer with a shared segment to transfer changes to devdraw this made development easier but the moment I had concurrent access my software crashed. Debugging made clear the source of the crash was the described call combination while using shared memory segments.
If there is interest for reproducing the exact circumstances I can write a small example app which involves different processes accessing the same shared memory segments which are inherited by the rfork methods.