Do I assume correctly that because the new thread stuff (including the
higher performance mutexes) will not be MFC'd to RELENG_5 because they
break the ABI?
In general, the documented ABI boundary for applications is the libc
interface, and not the kernel system call interface, so subject to some
limitations during the upgrade process, there's a lot of scope for changes
to the internals of thread implementations without breaking the
application ABI. A nice example of this is that using libmap.conf, you
can change between libthr, libpthread, and libc_r without breaking
anything in the appplication. So subject to careful management of the
various ABIs and interfaces, merging new thread work shouldn't be a
prohibited for ABI reasons.