GTK: http://build.webkit.org/results/GTK%20Linux%2064-bit%20Debug/r121089%20%2834292%29/results.html Qt: http://build.webkit.sed.hu/results/x86-64%20Linux%20Qt%20Debug/r121058%20%2823500%29/results.html Could you check it, please?
The reason this happens is because whatever is calling into JSC doesn't take a JSLock before doing so. This is expected behavior and it indicates a bug in the code that is calling into JSC.
fixed patch relanded - http://trac.webkit.org/changeset/121381