[dev.garbage] runtime: Stop running gs during the GCscan phase.

Ensure that all gs are in a scan state when their stacks are being scanned.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://golang.org/cl/179160044

src/runtime/mgc.go

@@ -923,14 +923,11 @@ func scanframe(frame *stkframe, unused unsafe.Pointer) bool {
 }
 
 func scanstack(gp *g) {
-	// TODO(rsc): Due to a precedence error, this was never checked in the original C version.
-	// If you enable the check, the gothrow happens.
-	/*
-		if readgstatus(gp)&_Gscan == 0 {
-			print("runtime: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", readgstatus(gp), "\n")
-			gothrow("mark - bad status")
-		}
-	*/
+
+	if readgstatus(gp)&_Gscan == 0 {
+		print("runtime:scanstack: gp=", gp, ", goid=", gp.goid, ", gp->atomicstatus=", hex(readgstatus(gp)), "\n")
+		gothrow("scanstack - bad status")
+	}
 
 	switch readgstatus(gp) &^ _Gscan {
 	default:

src/runtime/proc1.go

@@ -316,6 +316,10 @@ func casfrom_Gscanstatus(gp *g, oldval, newval uint32) {
 
 	// Check that transition is valid.
 	switch oldval {
+	default:
+		print("runtime: casfrom_Gscanstatus bad oldval gp=", gp, ", oldval=", hex(oldval), ", newval=", hex(newval), "\n")
+		dumpgstatus(gp)
+		gothrow("casfrom_Gscanstatus:top gp->status is not in scan state")
 	case _Gscanrunnable,
 		_Gscanwaiting,
 		_Gscanrunning,
@@ -417,13 +421,6 @@ func stopg(gp *g) bool {
 			return false
 
 		case _Grunning:
-			if gcphase == _GCscan {
-				// Running routines not scanned during
-				// GCscan phase, we only scan non-running routines.
-				gp.gcworkdone = true
-				return false
-			}
-
 			// Claim goroutine, so we aren't racing with a status
 			// transition away from Grunning.
 			if !castogscanstatus(gp, _Grunning, _Gscanrunning) {

src/runtime/stack1.go

@@ -682,7 +682,12 @@ func newstack() {
 			gothrow("runtime: g is running but p is not")
 		}
 		if gp.preemptscan {
+			for !castogscanstatus(gp, _Gwaiting, _Gscanwaiting) {
+				// Likely to be racing with the GC as it sees a _Gwaiting and does the stack scan.
+				// If so this stack will be scanned twice which does not change correctness.
+			}
 			gcphasework(gp)
+			casfrom_Gscanstatus(gp, _Gscanwaiting, _Gwaiting)
 			casgstatus(gp, _Gwaiting, _Grunning)
 			gp.stackguard0 = gp.stack.lo + _StackGuard
 			gp.preempt = false