runtime: rename gosweepdone to isSweepDone and document better

gosweepdone is another anachronism from the time when the sweeper was
implemented in C. Rename it to "isSweepDone" for the modern era.

Change-Id: I8472aa6f52478459c3f2edc8a4b2761e73c4c2dd
Reviewed-on: https://go-review.googlesource.com/c/138658
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/mgc.go

@@ -789,7 +789,7 @@ func gcSetTriggerRatio(triggerRatio float64) {
 		trigger = uint64(float64(memstats.heap_marked) * (1 + triggerRatio))
 		// Don't trigger below the minimum heap size.
 		minTrigger := heapminimum
-		if !gosweepdone() {
+		if !isSweepDone() {
 			// Concurrent sweep happens in the heap growth
 			// from heap_live to gc_trigger, so ensure
 			// that concurrent sweep has some heap growth
@@ -834,7 +834,7 @@ func gcSetTriggerRatio(triggerRatio float64) {
 	}
 
 	// Update sweep pacing.
-	if gosweepdone() {
+	if isSweepDone() {
 		mheap_.sweepPagesPerByte = 0
 	} else {
 		// Concurrent sweep needs to sweep all of the in-use

src/runtime/mgcsweep.go

@@ -60,7 +60,7 @@ func bgsweep(c chan int) {
 			Gosched()
 		}
 		lock(&sweep.lock)
-		if !gosweepdone() {
+		if !isSweepDone() {
 			// This can happen if a GC runs between
 			// gosweepone returning ^0 above
 			// and the lock being acquired.
@@ -134,8 +134,13 @@ func sweepone() uintptr {
 	return npages
 }
 
-//go:nowritebarrier
-func gosweepdone() bool {
+// isSweepDone reports whether all spans are swept or currently being swept.
+//
+// Note that this condition may transition from false to true at any
+// time as the sweeper runs. It may transition from true to false if a
+// GC runs; to prevent that the caller must be non-preemptible or must
+// somehow block GC progress.
+func isSweepDone() bool {
 	return mheap_.sweepdone != 0
 }