diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index b82569bb3e4..8eba5a8bc2d 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -333,6 +333,12 @@ func gc(mode int) {
 		var gcw gcWork
 		gcDrain(&gcw)
 		gcw.dispose()
+		// Despite the barrier in gcDrain, gcDrainNs may still
+		// be doing work at this point. This is okay because
+		// 1) the gcDrainNs happen on the system stack, so
+		// they will flush their work to the global queues
+		// before we can stop the world, and 2) it's fine if
+		// we go into mark termination with some work queued.
 
 		// Begin mark termination.
 		gctimer.cycle.markterm = nanotime()
diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 197b6a808da..8e0a88f0a36 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -366,6 +366,9 @@ func gcDrain(gcw *gcWork) {
 }
 
 // gcDrainN scans n objects, blackening grey objects.
+//
+// This MUST be run on the system stack to prevent a stop-the-world
+// while this locally holds GC work buffers.
 //go:nowritebarrier
 func gcDrainN(gcw *gcWork, n int) {
 	checknocurrentwbuf()