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.

1 files changed, 23 insertions, 22 deletions

diff --git a/sys/src/9/port/alarm.c b/sys/src/9/port/alarm.c
index b78088665..8fb45be94 100644
--- a/sys/src/9/port/alarm.c
+++ b/sys/src/9/port/alarm.c
@@ -11,7 +11,7 @@ void
 alarmkproc(void*)
 {
 	Proc *rp;
-	ulong now;
+	ulong now, when;
 
 	while(waserror())
 		;
@@ -19,20 +19,20 @@ alarmkproc(void*)
 	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 @@ void
 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 @@ procalarm(ulong time)
 	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;