runtime: reset gcscanvalid and gcworkdone when GODEBUG=gctrace=2

When GODEBUG=gctrace=2 two gcs are preformed. During the first gc
the stack scan sets the g's gcscanvalid and gcworkdone flags to true
indicating that the stacks have to be scanned and do not need to
be rescanned. These need to be reset to false for the second GC so the
stacks are rescanned, otherwise if the only pointer to an object is
on the stack it will not be discovered and the object will be freed.
Typically this will include the object that was just allocated in
the mallocgc call that initiated the GC.

Change-Id: Ic25163f4689905fd810c90abfca777324005c02f
Reviewed-on: https://go-review.googlesource.com/5861Reviewed-by: Russ Cox <rsc@golang.org>

src/runtime/mgc.go

@@ -339,7 +339,7 @@ func gc(mode int) {
 		})
 	} else {
 		// For non-concurrent GC (mode != gcBackgroundMode)
-		// g stack have not been scanned so set gcscanvalid
+		// The g stacks have not been scanned so set gcscanvalid
 		// such that mark termination scans all stacks.
 		// No races here since we are in a STW phase.
 		for _, gp := range allgs {
@@ -381,6 +381,14 @@ func gc(mode int) {
 
 		if debug.gctrace > 1 {
 			startTime = nanotime()
+			// 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
+			}
 			finishsweep_m()
 			gcMark(startTime)
 			gcSweep(mode)