add libc specific __cmtx and __rcmtx locks for internal use.

__cmtx provides mutual exclusion using futex(2) and cas on archs
that support it, or _spinlocks on the rest. __rcmtx is a recursive
mutex built on top of __cmtx, so it inherits the use of futex and
cas/spinlock from __cmtx.

until now the options we had for locking between threads in libc
were spinlocks or pthread mutexes. spinlocks use sched_yield to
relax if they have to spin on a contended lock, which we are trying
to minimise the use of as much as possible. pthread_mutex is
relatively large in memory and offers a lot of complicated features
which are unecessary for most of libc. the non cas/futex version
of pthread_mutexes currently relies on __thrsleep and __thrwakeup,
which we also want to deprecate.

having a small futex based lock available everywhere will help us
move away from overuse of spinlocks, and deprecate __thrsleep and
__thrwakeup.

ok kettenis@
jca@ provided his eyes too.

0 files changed, 0 insertions, 0 deletions