Notes for lwp on top of pthreads
--------------------------------

The pthread-lwp implementation is compatible with the non-preemptive
lwp implementation. It even encapsulates enough of the `quirks' to
allow venus to run without problems.


New function
------------

void PRE_Concurrent(int on)
/* on != 0 : Take the calling thread out of the LWP runqueues and let it
 *           run concurrently.
 * on == 0 : Return the calling thread under the reign of the big lock.
 */


Special notes
-------------

LWP RW-locks
  Can safely be used by both concurrent and non-concurrent threads.
  When locking, non-concurrent threads release the run_mutex.

LWP_QSignal/LWP_QWait
  Implemented as a counting semaphore. No more lost signals, and
  allows concurrent threads to use it without waiting for the run_mutex
  (yielding non-concurrent threads). Non-concurrent threads release the
  mutex while waiting.

LWP_SignalProcess/LWP_NoYieldSignal/LWP_MwaitProcess/LWP_WaitProcess
  Concurrent threads need to aquire the run_mutex and will therefore
  block until the current non-concurrent thread yields. This affects
  both waiting and signalling threads.


Compiling
---------

All c/c++ code which will be linked against liblwp_pt has to be compiled
with -D_REENTRANT.


Future additions
----------------

IOMGR_Read/IOMGR_Write functions that release the run_mutex for
non-concurrent threads (simple, look at IOMGR_Select). But they should
then also be implemented as `polling' functions in the normal LWP
implementation.


For more information, read the source (lwp_pt.c, lock_pt.c, etc.) I've
tried to put in a lot of useful comments.

-- Jan