runtime: make p.gcBgMarkWorker a guintptr

Currently p.gcBgMarkWorker is a *g. Change it to a guintptr. This
eliminates a write barrier during the subtle mark worker parking dance
(which isn't known to be causing problems, but may).

Change-Id: Ibf12c05ac910820448059e69a68e5b882c993ed8
Reviewed-on: https://go-review.googlesource.com/18970
Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Rick Hudson <rlh@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -629,7 +629,7 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
 	if gcBlackenEnabled == 0 {
 		throw("gcControllerState.findRunnable: blackening not enabled")
 	}
-	if _p_.gcBgMarkWorker == nil {
+	if _p_.gcBgMarkWorker == 0 {
 		// The mark worker associated with this P is blocked
 		// performing a mark transition. We can't run it
 		// because it may be on some other run or wait queue.
@@ -711,7 +711,7 @@ func (c *gcControllerState) findRunnableGCWorker(_p_ *p) *g {
 	}
 
 	// Run the background mark worker
-	gp := _p_.gcBgMarkWorker
+	gp := _p_.gcBgMarkWorker.ptr()
 	casgstatus(gp, _Gwaiting, _Grunnable)
 	if trace.enabled {
 		traceGoUnpark(gp, 0)
@@ -1325,7 +1325,7 @@ func gcBgMarkStartWorkers() {
 		if p == nil || p.status == _Pdead {
 			break
 		}
-		if p.gcBgMarkWorker == nil {
+		if p.gcBgMarkWorker == 0 {
 			go gcBgMarkWorker(p)
 			notetsleepg(&work.bgMarkReady, -1)
 			noteclear(&work.bgMarkReady)
@@ -1356,11 +1356,6 @@ func gcBgMarkWorker(_p_ *p) {
 	}
 	var park parkInfo
 
-	// casgp is casp for *g's.
-	casgp := func(gpp **g, old, new *g) bool {
-		return casp((*unsafe.Pointer)(unsafe.Pointer(gpp)), unsafe.Pointer(old), unsafe.Pointer(new))
-	}
-
 	gp := getg()
 	park.m = acquirem()
 	park.attach = _p_
@@ -1397,7 +1392,7 @@ func gcBgMarkWorker(_p_ *p) {
 				// cas the worker because we may be
 				// racing with a new worker starting
 				// on this P.
-				if !casgp(&p.gcBgMarkWorker, nil, g) {
+				if !p.gcBgMarkWorker.cas(0, guintptr(unsafe.Pointer(g))) {
 					// The P got a new worker.
 					// Exit this worker.
 					return false
@@ -1409,7 +1404,7 @@ func gcBgMarkWorker(_p_ *p) {
 		// Loop until the P dies and disassociates this
 		// worker (the P may later be reused, in which case
 		// it will get a new worker) or we failed to associate.
-		if _p_.gcBgMarkWorker != gp {
+		if _p_.gcBgMarkWorker.ptr() != gp {
 			break
 		}
 
@@ -1478,7 +1473,7 @@ func gcBgMarkWorker(_p_ *p) {
 			// as the worker for this P so
 			// findRunnableGCWorker doesn't try to
 			// schedule it.
-			_p_.gcBgMarkWorker = nil
+			_p_.gcBgMarkWorker.set(nil)
 			releasem(park.m)
 
 			gcMarkDone()

src/runtime/proc.go

@@ -1867,9 +1867,9 @@ stop:
 	// We have nothing to do. If we're in the GC mark phase, can
 	// safely scan and blacken objects, and have work to do, run
 	// idle-time marking rather than give up the P.
-	if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != nil && gcMarkWorkAvailable(_p_) {
+	if _p_ := _g_.m.p.ptr(); gcBlackenEnabled != 0 && _p_.gcBgMarkWorker != 0 && gcMarkWorkAvailable(_p_) {
 		_p_.gcMarkWorkerMode = gcMarkWorkerIdleMode
-		gp := _p_.gcBgMarkWorker
+		gp := _p_.gcBgMarkWorker.ptr()
 		casgstatus(gp, _Gwaiting, _Grunnable)
 		if trace.enabled {
 			traceGoUnpark(gp, 0)
@@ -3206,15 +3206,15 @@ func procresize(nprocs int32) *p {
 		}
 		// if there's a background worker, make it runnable and put
 		// it on the global queue so it can clean itself up
-		if p.gcBgMarkWorker != nil {
-			casgstatus(p.gcBgMarkWorker, _Gwaiting, _Grunnable)
+		if gp := p.gcBgMarkWorker.ptr(); gp != nil {
+			casgstatus(gp, _Gwaiting, _Grunnable)
 			if trace.enabled {
-				traceGoUnpark(p.gcBgMarkWorker, 0)
+				traceGoUnpark(gp, 0)
 			}
-			globrunqput(p.gcBgMarkWorker)
+			globrunqput(gp)
 			// This assignment doesn't race because the
 			// world is stopped.
-			p.gcBgMarkWorker = nil
+			p.gcBgMarkWorker.set(nil)
 		}
 		for i := range p.sudogbuf {
 			p.sudogbuf[i] = nil

src/runtime/runtime2.go

@@ -388,7 +388,7 @@ type p struct {
 
 	// Per-P GC state
 	gcAssistTime     int64 // Nanoseconds in assistAlloc
-	gcBgMarkWorker   *g
+	gcBgMarkWorker   guintptr
 	gcMarkWorkerMode gcMarkWorkerMode
 
 	// gcw is this P's GC work buffer cache. The work buffer is