xref: /src/lib/libpthread/README

This pthread package is/will be based on the POSIX1003.4a Draft 7 pthread
standard, and Frank Mullers paper on signal handelling presented
at the Winter 93 USENIX conference.

It is currently being designed and written by me, Chris Provenzano.
All bug, comments, and questions can be sent me at either
proven@athena.mit.edu or proven@sun-lamp.cs.berkeley.edu
Please don't send bugs or patches to any of the NetBSD mailing lists.

Thanks goes to John Carr jfc@mit.edu for porting this to the IBM/RT,
and for his bug reports and fixes.

PORTING
One of the goals of this user space implementation of pthreads is that it
be portable. I have minimized the ammount of assembler code necessary,
but some is.

If you want to port it to another platform here are a few basic hints.

There are currently three files you'll have to creat for your
architecture, machdep.h, machdep.c and syscall.S.
The first two are necessary to get the context switch section of
the pthread package running, the third is for all the syscalls.

To do an initial port, create an appropriate machdep.h, and machdep.c
and define PTHREAD_INITIAL_PORT in the Makefile

Comment out references to the stdio package.

INCLUDE FILES AND PORTING
To continue to make this package portable, some basic rules on includes
files must be followed.

pthread.h should be included first (if it is to be included).
machdep.h should define size_t if the system doesn't define it already

stdio.h should not be included. It is included in pthread.h.

posix.h should be included last. This file is used to correct non
POSIX features, after everything else has been defined.

INTERNAL LOCKING
To prevent deadlocks the following rules were used for locks.

1.	Local locks for mutex queues and other like things are only locked
	by running threads, at NO time will a local lock be held by
	a thread in a non running state.
2.  Only threads that are in a run state can attempt to lock another thread,
	this way, we can assume that the lock will be released shortly, and don't
	have to unlock the local lock.
3.	The only time a thread will have a pthread->lock and is not in a run
	state is when it is in the reschedule routine.
4.	The reschedule routine assumes all local locks have been released,
	there is a lock on the currently running thread (pthread_run),
	and that this thread is being rescheduled to a non running state.
	It is safe to unlock the currently running threads lock after it
	has been rescheduled.
5.	The reschedule routine locks the kernel, sets the state of the currently
	running thread, unlocks the currently running thread, calls the
	context switch routines.
6	the kernel lock is used only ...


7.	The order of locking is ...

1 local locks
2 pthread->lock			/* Assumes it will get it soon */
3 pthread_run->lock		/* Assumes it will get it soon, but must release 2 */
4 kernel lock			/* Currently assumes it will ALWAYS get it. */

8.	The kernel lock will be changed to a spin lock for systems that
already support kernel threads, this way we can mutiplex threads onto
kernel threads.
9.	There are points where the kernel is locked and it needs to get
either a local lock or a pthread lock, if at these points the code
fails to get the lock the kernel gives up and sets a flag which will
be checked at a later point.
10.	Interrupts are dissabled while the kernel is locked, the interrupt
mask must be checked afterwards or cleared in some way, after interrputs
have been reenabled, this allows back to back interrupts, but should always
avoid missing one.

Copyright (c) 1993 Chris Provenzano. All rights reserved.

This product includes software developed by the Univeristy of California,
Berkeley and its contributors.