diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 2bc3c920dc..d1f5088b50 100644
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1602,11 +1602,19 @@ func startm(_p_ *p, spinning bool) {
 // Always runs without a P, so write barriers are not allowed.
 //go:nowritebarrier
 func handoffp(_p_ *p) {
+	// handoffp must start an M in any situation where
+	// findrunnable would return a G to run on _p_.
+
 	// if it has local work, start it straight away
 	if !runqempty(_p_) || sched.runqsize != 0 {
 		startm(_p_, false)
 		return
 	}
+	// if it has GC work, start it straight away
+	if gcBlackenEnabled != 0 && gcMarkWorkAvailable(_p_) {
+		startm(_p_, false)
+		return
+	}
 	// no local work, check that there are no spinning/idle M's,
 	// otherwise our help is not required
 	if atomic.Load(&sched.nmspinning)+atomic.Load(&sched.npidle) == 0 && atomic.Cas(&sched.nmspinning, 0, 1) { // TODO: fast atomic
@@ -1787,6 +1795,10 @@ func execute(gp *g, inheritTime bool) {
 func findrunnable() (gp *g, inheritTime bool) {
 	_g_ := getg()
 
+	// The conditions here and in handoffp must agree: if
+	// findrunnable would return a G to run, handoffp must start
+	// an M.
+
 top:
 	if sched.gcwaiting != 0 {
 		gcstopm()