Why does this problem not appear under FreeBSD?
It does happen in FreeBSD, but you have to catch it at the right
time. Since FreeBSD is effectively single-threaded with their
mutual exclusion locks, this happens much less in FreeBSD. However
it also means when we both release the big kernel lock, we'll be
able to scale to much higher number of processors because we're
multi-threaded instead of single-threaded in the kernel.
Is it because of the multi-threaded kernel?
Yes.