Reproducible on ruby 2.2.0dev (2014-04-09 trunk 45543) [x86_64-linux]
Not reproducible on ruby 2.0.0p451 [x86_64-linux]

I tried the pure Ruby thread.rb from 2.0.0 with trunk, but the segfault
still happens, so I do not think this is a problem with the new thread.so

Looks like a GC problem, valgrind -v output: http://yhbt.net/9718.txt
I'm stuck for now :<