runtime: consolidate gcworkdone/gcscanvalid clearing loops

Previously, we had three loops in the garbage collector that all
cleared the per-G GC flags.  Consolidate these into one function.
This one function is designed to work in a concurrent setting.  As a
result, it's slightly more expensive than the loops it replaces during
STW phases, but these happen at most twice per GC.

Change-Id: Id1ec0074fd58865eb0112b8a0547b267802d0df1
Reviewed-on: https://go-review.googlesource.com/5881Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/mgc.go

@@ -339,13 +339,9 @@ func gc(mode int) {
 		})
 	} else {
 		// For non-concurrent GC (mode != gcBackgroundMode)
-		// The g stacks have not been scanned so set gcscanvalid
+		// The g stacks have not been scanned so clear g state
 		// such that mark termination scans all stacks.
-		// No races here since we are in a STW phase.
-		for _, gp := range allgs {
-			gp.gcworkdone = false  // set to true in gcphasework
-			gp.gcscanvalid = false // stack has not been scanned
-		}
+		gcResetGState()
 	}
 
 	startTime := nanotime()
@@ -384,11 +380,7 @@ func gc(mode int) {
 			// The g stacks have been scanned so
 			// they have gcscanvalid==true and gcworkdone==true.
 			// Reset these so that all stacks will be rescanned.
-			// No races here since we are in a STW phase.
-			for _, gp := range allgs {
-				gp.gcworkdone = false  // set to true in gcphasework
-				gp.gcscanvalid = false // stack has not been scanned
-			}
+			gcResetGState()
 			finishsweep_m()
 			gcMark(startTime)
 			gcSweep(mode)
@@ -606,6 +598,22 @@ func gcCopySpans() {
 	unlock(&mheap_.lock)
 }
 
+// gcResetGState resets the GC state of all G's and returns the length
+// of allgs.
+func gcResetGState() int {
+	// This may be called during a concurrent phase, so make sure
+	// allgs doesn't change.
+	lock(&allglock)
+	local_allglen := allglen
+	for i := uintptr(0); i < local_allglen; i++ {
+		gp := allgs[i]
+		gp.gcworkdone = false  // set to true in gcphasework
+		gp.gcscanvalid = false // stack has not been scanned
+	}
+	unlock(&allglock)
+	return int(local_allglen)
+}
+
 // Hooks for other packages
 
 var poolcleanup func()

src/runtime/mgcmark.go

@@ -27,14 +27,7 @@ func gcscan_m() {
 	// At the bottom we will want to return this p back to the scheduler.
 
 	// Prepare flag indicating that the scan has not been completed.
-	lock(&allglock)
-	local_allglen := allglen
-	for i := uintptr(0); i < local_allglen; i++ {
-		gp := allgs[i]
-		gp.gcworkdone = false  // set to true in gcphasework
-		gp.gcscanvalid = false // stack has not been scanned
-	}
-	unlock(&allglock)
+	local_allglen := gcResetGState()
 
 	work.nwait = 0
 	work.ndone = 0
@@ -45,7 +38,7 @@ func gcscan_m() {
 
 	lock(&allglock)
 	// Check that gc work is done.
-	for i := uintptr(0); i < local_allglen; i++ {
+	for i := 0; i < local_allglen; i++ {
 		gp := allgs[i]
 		if !gp.gcworkdone {
 			throw("scan missed a g")