forsyth posted to 9fans a fix for privfree not working.  

a few trivial changes
- remove privinit
- add paranoid check in privfree to avoid undefined shift

Reference: /n/atom/patch/applied/privallocfix
Date: Mon Sep  7 17:19:58 CES 2015
Signed-off-by: quanstro@quanstro.net

--- /sys/src/libc/9sys/privalloc.c	Mon Sep  7 17:16:55 2015
+++ /sys/src/libc/9sys/privalloc.c	Mon Sep  7 17:16:56 2015
@@ -2,8 +2,7 @@
 #include <libc.h>
 
 static Lock	privlock;
-static int	privinit;
-static void	**privs;
+static u32int privmap;
 
 extern void	**_privates;
 extern int	_nprivates;
@@ -15,31 +14,30 @@
 	int i;
 
 	lock(&privlock);
-	if(!privinit){
-		privinit = 1;
-		if(_nprivates){
-			_privates[0] = 0;
-			for(i = 1; i < _nprivates; i++)
-				_privates[i] = &_privates[i - 1];
-			privs = &_privates[i - 1];
+	for(i = 0; i < 32 && i < _nprivates; i++){
+		if((privmap & (1<<i)) == 0){
+			privmap |= 1<<i;
+			unlock(&privlock);
+			p = &_privates[i];
+			*p = nil;
+			return p;
 		}
 	}
-	p = privs;
-	if(p != nil){
-		privs = *p;
-		*p = nil;
-	}
 	unlock(&privlock);
-	return p;
+	return nil;
 }
 
 void
 privfree(void **p)
 {
-	lock(&privlock);
-	if(p != nil && privinit){
-		*p = privs;
-		privs = p;
+	int i;
+
+	if(p != nil){
+		i = p - _privates;
+		if(i < 0 || i >= 32 || i >= _nprivates || (privmap & (1<<i)) == 0)
+			abort();
+		lock(&privlock);
+		privmap &= ~(1<<i);
+		unlock(&privlock);
 	}
-	unlock(&privlock);
 }