Adds some fairness to altsems and fixes a bug.

Reference: /n/patches.lsub.org/patch/fairaltsems
Date: Sun Jul 15 13:53:07 CES 2012
Signed-off-by: esoriano@lsub.org

--- /sys/src/nix/port/syssem.c	Thu Jul  5 11:00:28 2012
+++ /sys/src/nix/port/syssem.c	Thu Jul 12 16:07:21 2012
@@ -280,10 +280,11 @@
 	semwakeup(s);
 }
 
-
+static int nextindex;
 /*
  * Alt makes its best efford to get a token from any sem in the array.
  * It ignores the dead sems and only crashes if all sems are dead.
+ * Nextstart is used to prevent starvation.
  */
 static int
 semalt(Ksem *ss[], int n)
@@ -292,10 +293,13 @@
 	Ksem *s;
 	RWlock *rwl;
 	int queued;
+	ulong from;
 
 	if(n < 1)
 		error(Ebadarg);
 
+	from = (ulong)semainc(&nextindex) % n;
+
 	/*
 	 * While searching an available sem, the proc
 	 * should not be awaken in a previously processed sem.
@@ -306,11 +310,11 @@
 	up->waitsem = nil;
 	up->semawaken = 0;
 	queued = 0;
-
+	
 	for(i = 0; i < n; i++){
-		s = ss[i];
+		s = ss[(from+i)%n];
 		/*
-		 * if the sem is dead, ignore it an keep searching
+		 * if the sem is dead, ignore it and keep searching
 		 */
 		if(waserror())	
 			continue;
@@ -321,7 +325,6 @@
 			poperror();
 			up->waitsem = s;
 			up->semawaken = 1;
-			i++;
 			wunlock(rwl);
 			goto Done;
 		}
@@ -352,11 +355,17 @@
 		/*
 		 * We are probably being killed.
 		 */
-		for(i  = 0; i < n; i++){	
+		for(i = 0; i < n; i++){	
 			s = ss[i];
 			lock(s);
-			if(semdequeueme(s) != 0)
-				panic("semalt: the proc is not in the queue");
+			/*
+			 * up->waitsem was nil, but there can be dead sems,
+			 * so semdequeueme could return -1.
+			 */
+			if(semdequeueme(s) < 0){
+				unlock(s);
+				continue;
+			}
 			unlock(s);
 			if(! waserror()){
 				_userlock(s);
@@ -372,7 +381,7 @@
 Done:
 	DBG("semalt up %#p awaken\n", up);
 	r = -1;
-	for(j = 0; j < i; j++){
+	for(j = 0; j < n; j++){
 		s = ss[j];	
 		if(s == up->waitsem)
 			r = j;
@@ -380,7 +389,8 @@
 			/*
 			 * Cancel the reservation for the sem.
 			 * Note that the sem could be already
-			 * dequeued by semwakeup.
+			 * dequeued by semwakeup or never
+			 * queued in this sem.
 			 */
 			lock(s);
 			if(semdequeueme(s) == 0){
@@ -496,18 +506,23 @@
 altsems(Ksem *ss[], int n)
 {
 	int i, w;
+	ulong p;
+
+	p = (ulong) ainc(&nextindex);
 
+	i = 0;
 	/* busy wait */
 	for(w = 0; w < Semtrytimes; w++){
 		for(i = 0; i < n; i++)
-			if(ss[i]->sem->tickets > 0)
+			if(ss[(p+i)%n]->sem->tickets > 0)
 				break;
 		if(i < n)
 			break;
 	}
+	p = (p+i)%n;
 	for(i = 0; i < n; i++)
-		if(downsem(ss[i], 0) == 1)
-			return i;
+		if(downsem(ss[(p+i)%n], 0) == 1)
+			return (p+i)%n;
 
 	return semalt(ss, n);
 }
--- /sys/src/libc/9sys/upsem.c	Wed Jul  4 11:43:12 2012
+++ /sys/src/libc/9sys/upsem.c	Thu Jul 12 11:04:09 2012
@@ -101,31 +101,37 @@
 	return 1;
 }
 
-/*
- * No optimistic for now.
- */
+static int nextindex;
+
 int
 altsems(Sem *ss[], int n)
 {
-	int  w, i, j;
+	int  w, i;
+	ulong p;
 
-	i = 0;
 	if(ss == nil || n <= 0){
 		werrstr("altsems: bad args");
 		return -1;
 	}
+
+	i = 0;
+	p = (ulong) ainc(&nextindex);
+
 	/* busy wait */
 	for(w = 0; w < semtrytimes; w++){
 		for(i = 0; i < n; i++)
-			if(ss[i]->tickets > 0)
+			if(ss[(p+i)%n]->tickets > 0)
 				break;
 		if(i < n)
 			break;
 	}
-	for(j = 0; j < n; j++)
-		if(downsem(ss[(i+j)%n], 0) == 1){
+	
+	p = (p+i)%n;
+
+	for(i = 0; i < n; i++)
+		if(downsem(ss[(p+i)%n], 0) == 1){
 			ainc(&c.ualt);
-			return (i+j)%n;
+			return (p+i)%n;
 		}
 	ainc(&c.kalt);
 	return semalt(ss, n);