From 40a9ee95108885c4a33fc035e0f17f46af9749de Mon Sep 17 00:00:00 2001
From: rpj <rpj>
Date: Mon, 4 Jun 2001 03:03:12 +0000
Subject: * condvar.c: Add original description of the algorithm as
developed by Terekhov and Thomas, plus reference to README.CV.
---
condvar.c | 126 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 126 insertions(+)
diff --git a/condvar.c b/condvar.c
index eb1fd9a..3607594 100644
--- a/condvar.c
+++ b/condvar.c
@@ -4,6 +4,132 @@
* Description:
* This translation unit implements condition variables and their primitives.
*
+ * Algorithm:
+ * The algorithm used in this implementation is that developed by
+ * Alexander Terekhov in colaboration with Louis Thomas. The bulk
+ * of the discussion is recorded in the file README.CV, which contains
+ * several generations of both colaborators original algorithms. The final
+ * algorithm used here is the one referred to as
+ *
+ * Algorithm 8a / IMPL_SEM,UNBLOCK_STRATEGY == UNBLOCK_ALL
+ *
+ * presented below in pseudo-code as it appeared:
+ *
+ *
+ * given:
+ * semBlockLock - bin.semaphore
+ * semBlockQueue - semaphore
+ * mtxExternal - mutex or CS
+ * mtxUnblockLock - mutex or CS
+ * nWaitersGone - int
+ * nWaitersBlocked - int
+ * nWaitersToUnblock - int
+ *
+ * wait( timeout ) {
+ *
+ * [auto: register int result ] // error checking omitted
+ * [auto: register int nSignalsWasLeft ]
+ * [auto: register int nWaitersWasGone ]
+ *
+ * sem_wait( semBlockLock );
+ * nWaitersBlocked++;
+ * sem_post( semBlockLock );
+ *
+ * unlock( mtxExternal );
+ * bTimedOut = sem_wait( semBlockQueue,timeout );
+ *
+ * lock( mtxUnblockLock );
+ * if ( 0 != (nSignalsWasLeft = nWaitersToUnblock) ) {
+ * if ( bTimeout ) { // timeout (or canceled)
+ * if ( 0 != nWaitersBlocked ) {
+ * nWaitersBlocked--;
+ * }
+ * else {
+ * nWaitersGone++; // count spurious wakeups.
+ * }
+ * }
+ * if ( 0 == --nWaitersToUnblock ) {
+ * if ( 0 != nWaitersBlocked ) {
+ * sem_post( semBlockLock ); // open the gate.
+ * nSignalsWasLeft = 0; // do not open the gate
+ * // below again.
+ * }
+ * else if ( 0 != (nWaitersWasGone = nWaitersGone) ) {
+ * nWaitersGone = 0;
+ * }
+ * }
+ * }
+ * else if ( INT_MAX/2 == ++nWaitersGone ) { // timeout/canceled or
+ * // spurious semaphore :-)
+ * sem_wait( semBlockLock );
+ * nWaitersBlocked -= nWaitersGone; // something is going on here
+ * // - test of timeouts? :-)
+ * sem_post( semBlockLock );
+ * nWaitersGone = 0;
+ * }
+ * unlock( mtxUnblockLock );
+ *
+ * if ( 1 == nSignalsWasLeft ) {
+ * if ( 0 != nWaitersWasGone ) {
+ * // sem_adjust( semBlockQueue,-nWaitersWasGone );
+ * while ( nWaitersWasGone-- ) {
+ * sem_wait( semBlockQueue ); // better now than spurious later
+ * }
+ * } sem_post( semBlockLock ); // open the gate
+ * }
+ *
+ * lock( mtxExternal );
+ *
+ * return ( bTimedOut ) ? ETIMEOUT : 0;
+ * }
+ *
+ * signal(bAll) {
+ *
+ * [auto: register int result ]
+ * [auto: register int nSignalsToIssue]
+ *
+ * lock( mtxUnblockLock );
+ *
+ * if ( 0 != nWaitersToUnblock ) { // the gate is closed!!!
+ * if ( 0 == nWaitersBlocked ) { // NO-OP
+ * return unlock( mtxUnblockLock );
+ * }
+ * if (bAll) {
+ * nWaitersToUnblock += nSignalsToIssue=nWaitersBlocked;
+ * nWaitersBlocked = 0;
+ * }
+ * else {
+ * nSignalsToIssue = 1;
+ * nWaitersToUnblock++;
+ * nWaitersBlocked--;
+ * }
+ * }
+ * else if ( nWaitersBlocked > nWaitersGone ) { // HARMLESS RACE CONDITION!
+ * sem_wait( semBlockLock ); // close the gate
+ * if ( 0 != nWaitersGone ) {
+ * nWaitersBlocked -= nWaitersGone;
+ * nWaitersGone = 0;
+ * }
+ * if (bAll) {
+ * nSignalsToIssue = nWaitersToUnblock = nWaitersBlocked;
+ * nWaitersBlocked = 0;
+ * }
+ * else {
+ * nSignalsToIssue = nWaitersToUnblock = 1;
+ * nWaitersBlocked--;
+ * }
+ * }
+ * else { // NO-OP
+ * return unlock( mtxUnblockLock );
+ * }
+ *
+ * unlock( mtxUnblockLock );
+ * sem_post( semBlockQueue,nSignalsToIssue );
+ * return result;
+ * }
+ *
+ * -------------------------------------------------------------
+ *
* Pthreads-win32 - POSIX Threads Library for Win32
* Copyright (C) 1998
*
--
cgit v1.2.3