shithub: riscv

Download patch

ref: f481ac716d3fd3320ddf0672b395cf374a41a1dd
parent: 0feb6e06af1e48c2b1a937c6a593fceeafd728d2
author: cinap_lenrek <cinap_lenrek@felloff.net>
date: Wed Jan 1 00:14:38 EST 2014

kernel: fix alarm postnote race

when alarmkproc is commited to send the alarm note to the process,
the process might have exited already, or worse, being reused for
another process. pexit() zeros p->alarm at the beginning, but the
kalarmproc() might read p->alarm before pexit() zeroed it, decide
to send the note, then get preempted and pexit() releases the proc.
once kalarmproc() is resumed, the proc might be already something
different and we send the note to the wrong thing.

we now check p->alarm under the debug qlock. that way, pexit()
cannot make progress while we test the condition.

remove the error label arround postnote(). postnote does not raise
error.

--- a/sys/src/9/port/alarm.c
+++ b/sys/src/9/port/alarm.c
@@ -11,7 +11,7 @@
 alarmkproc(void*)
 {
 	Proc *rp;
-	ulong now;
+	ulong now, when;
 
 	while(waserror())
 		;
@@ -19,20 +19,20 @@
 	for(;;){
 		now = MACHP(0)->ticks;
 		qlock(&alarms);
-		while((rp = alarms.head) && (long)(now - rp->alarm) >= 0){
-			if(rp->alarm != 0L){
-				if(canqlock(&rp->debug)){
-					if(!waserror()){
-						postnote(rp, 0, "alarm", NUser);
-						poperror();
-					}
-					qunlock(&rp->debug);
-					rp->alarm = 0L;
-				}else
-					break;
+		for(rp = alarms.head; rp != nil; rp = rp->palarm){
+			if((when = rp->alarm) == 0)
+				continue;
+			if((long)(now - when) < 0)
+				break;
+			if(!canqlock(&rp->debug))
+				break;
+			if(rp->alarm != 0){
+				postnote(rp, 0, "alarm", NUser);
+				rp->alarm = 0;
 			}
-			alarms.head = rp->palarm;
+			qunlock(&rp->debug);
 		}
+		alarms.head = rp;
 		qunlock(&alarms);
 
 		sleep(&alarmr, return0, 0);
@@ -46,13 +46,15 @@
 checkalarms(void)
 {
 	Proc *p;
-	ulong now;
+	ulong now, when;
 
 	p = alarms.head;
-	now = MACHP(0)->ticks;
-
-	if(p != nil && (long)(now - p->alarm) >= 0)
-		wakeup(&alarmr);
+	if(p != nil){
+		now = MACHP(0)->ticks;
+		when = p->alarm;
+		if(when == 0 || (long)(now - when) >= 0)
+			wakeup(&alarmr);
+	}
 }
 
 ulong
@@ -61,10 +63,9 @@
 	Proc **l, *f;
 	ulong when, old;
 
-	if(up->alarm)
-		old = tk2ms(up->alarm - MACHP(0)->ticks);
-	else
-		old = 0;
+	old = up->alarm;
+	if(old)
+		old = tk2ms(old - MACHP(0)->ticks);
 	if(time == 0) {
 		up->alarm = 0;
 		return old;
--