diff --git a/src/runtime/mgc.go b/src/runtime/mgc.go
index c1edd17842..628a77fc1e 100644
--- a/src/runtime/mgc.go
+++ b/src/runtime/mgc.go
@@ -2099,9 +2099,14 @@ func clearpools() {
 	unlock(&sched.deferlock)
 }
 
-// Timing
-
-//go:nowritebarrier
+// gchelper runs mark termination tasks on Ps other than the P
+// coordinating mark termination.
+//
+// The caller is responsible for ensuring that this has a P to run on,
+// even though it's running during STW. Because of this, it's allowed
+// to have write barriers.
+//
+//go:yeswritebarrierrec
 func gchelper() {
 	_g_ := getg()
 	_g_.m.traceback = 2
@@ -2136,6 +2141,8 @@ func gchelperstart() {
 	}
 }
 
+// Timing
+
 // itoaDiv formats val/(10**dec) into buf.
 func itoaDiv(buf []byte, val uint64, dec int) []byte {
 	i := len(buf) - 1
diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index af9b33886c..8383eb51a1 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1938,7 +1938,9 @@ retry:
 	notesleep(&_g_.m.park)
 	noteclear(&_g_.m.park)
 	if _g_.m.helpgc != 0 {
+		// helpgc() set _g_.m.p and _g_.m.mcache, so we have a P.
 		gchelper()
+		// Undo the effects of helpgc().
 		_g_.m.helpgc = 0
 		_g_.m.mcache = nil
 		_g_.m.p = 0