kernel: fix twakeup()/timerdel() race condition

timerdel() did not make sure that the timer function
is not active (on another cpu). just acquiering the
Timer lock in the timer function only blocks the caller
of timerdel()/timeradd() but not the other way arround
(on a multiprocessor).

this changes the timer code to track activity of
the timer function, having timerdel() wait until
the timer has finished executing.

1 files changed, 7 insertions, 17 deletions

diff --git a/sys/src/9/port/proc.c b/sys/src/9/port/proc.c
index b39fc0942..e9a299c20 100644
--- a/sys/src/9/port/proc.c
+++ b/sys/src/9/port/proc.c
@@ -822,28 +822,17 @@ tfn(void *arg)
 	return up->trend == nil || up->tfn(arg);
 }
 
-static void
+void
 twakeup(Ureg*, Timer *t)
 {
 	Proc *p;
 	Rendez *trend;
 
-	ilock(t);
 	p = t->ta;
 	trend = p->trend;
 	if(trend != nil){
-		wakeup(trend);
 		p->trend = nil;
-	}
-	iunlock(t);
-}
-
-static void
-stoptimer(void)
-{
-	if(up->trend != nil){
-		up->trend = nil;
-		timerdel(up);
+		wakeup(trend);
 	}
 }
 
@@ -864,11 +853,13 @@ tsleep(Rendez *r, int (*fn)(void*), void *arg, ulong ms)
 	timeradd(up);
 
 	if(waserror()){
-		stoptimer();
+		up->trend = nil;
+		timerdel(up);
 		nexterror();
 	}
 	sleep(r, tfn, arg);
-	stoptimer();
+	up->trend = nil;
+	timerdel(up);
 	poperror();
 }
 
@@ -1102,8 +1093,7 @@ pexit(char *exitstr, int freemem)
 	void (*pt)(Proc*, int, vlong);
 
 	up->alarm = 0;
-	if(up->tt != nil)
-		timerdel(up);
+	timerdel(up);
 	pt = proctrace;
 	if(pt != nil)
 		pt(up, SDead, 0);