runtime: make test for freezetheworld more precise

exitsyscallfast checks for freezetheworld, but does so only by
checking if stopwait is positive. This can also happen during
stoptheworld, which is harmless, but confusing. Shortly, it will be
important that we get to the p.status cas even if stopwait is set.

Hence, make this test more specific so it only triggers with
freezetheworld and not other uses of stopwait.

Change-Id: Ibb722cd8360c3ed5a9654482519e3ceb87a8274d
Reviewed-on: https://go-review.googlesource.com/8205
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/proc1.go

@@ -208,6 +208,10 @@ func helpgc(nproc int32) {
 	unlock(&sched.lock)
 }
 
+// freezeStopWait is a large value that freezetheworld sets
+// sched.stopwait to in order to request that all Gs permanently stop.
+const freezeStopWait = 0x7fffffff
+
 // Similar to stoptheworld but best-effort and can be called several times.
 // There is no reverse operation, used during crashing.
 // This function must not lock any mutexes.
@@ -220,7 +224,7 @@ func freezetheworld() {
 	// so try several times
 	for i := 0; i < 5; i++ {
 		// this should tell the scheduler to not start any new goroutines
-		sched.stopwait = 0x7fffffff
+		sched.stopwait = freezeStopWait
 		atomicstore(&sched.gcwaiting, 1)
 		// this should stop running goroutines
 		if !preemptall() {
@@ -1864,7 +1868,7 @@ func exitsyscallfast() bool {
 	_g_ := getg()
 
 	// Freezetheworld sets stopwait but does not retake P's.
-	if sched.stopwait != 0 {
+	if sched.stopwait == freezeStopWait {
 		_g_.m.mcache = nil
 		_g_.m.p = nil
 		return false