runtime: use only per-P gcWork

Currently most uses of gcWork use the per-P gcWork, but there are two
places that still use a stack-based gcWork. Simplify things by making
these instead use the per-P gcWork.

Change-Id: I712d012cce9dd5757c8541824e9641ac1c2a329c
Reviewed-on: https://go-review.googlesource.com/19636
Reviewed-by: Rick Hudson <rlh@golang.org>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/runtime/mgc.go

@@ -1555,8 +1555,8 @@ func gcMark(start_time int64) {
 
 	gchelperstart()
 
-	var gcw gcWork
-	gcDrain(&gcw, gcDrainBlock)
+	gcw := &getg().m.p.ptr().gcw
+	gcDrain(gcw, gcDrainBlock)
 	gcw.dispose()
 
 	gcMarkRootCheck()
@@ -1798,8 +1798,8 @@ func gchelper() {
 
 	// Parallel mark over GC roots and heap
 	if gcphase == _GCmarktermination {
-		var gcw gcWork
-		gcDrain(&gcw, gcDrainBlock) // blocks in getfull
+		gcw := &_g_.m.p.ptr().gcw
+		gcDrain(gcw, gcDrainBlock) // blocks in getfull
 		gcw.dispose()
 	}
 

src/runtime/mgcwork.go

@@ -44,14 +44,6 @@ func (wp wbufptr) ptr() *workbuf {
 //
 // A gcWork can be used on the stack as follows:
 //
-//     var gcw gcWork
-//     disable preemption
-//     .. call gcw.put() to produce and gcw.get() to consume ..
-//     gcw.dispose()
-//     enable preemption
-//
-// Or from the per-P gcWork cache:
-//
 //     (preemption must be disabled)
 //     gcw := &getg().m.p.ptr().gcw
 //     .. call gcw.put() to produce and gcw.get() to consume ..