diff --git a/src/runtime/mgcmark.go b/src/runtime/mgcmark.go
index 8384190407..bad7c7e92b 100644
--- a/src/runtime/mgcmark.go
+++ b/src/runtime/mgcmark.go
@@ -602,9 +602,6 @@ func gcFlushBgCredit(scanWork int64) {
 //go:nowritebarrier
 func scanstack(gp *g) {
 	if gp.gcscanvalid {
-		if gcphase == _GCmarktermination {
-			gcRemoveStackBarriers(gp)
-		}
 		return
 	}
 
@@ -647,6 +644,7 @@ func scanstack(gp *g) {
 	} else {
 		sp = gp.sched.sp
 	}
+	gcLockStackBarriers(gp) // Not necessary during mark term, but harmless.
 	switch gcphase {
 	case _GCmark:
 		// Install stack barriers during stack scan.
@@ -657,16 +655,18 @@ func scanstack(gp *g) {
 			nextBarrier = ^uintptr(0)
 		}
 
-		if gp.stkbarPos != 0 || len(gp.stkbar) != 0 {
-			// If this happens, it's probably because we
-			// scanned a stack twice in the same phase.
-			print("stkbarPos=", gp.stkbarPos, " len(stkbar)=", len(gp.stkbar), " goid=", gp.goid, " gcphase=", gcphase, "\n")
-			throw("g already has stack barriers")
-		}
-
-		gcLockStackBarriers(gp)
+		// Remove any existing stack barriers before we
+		// install new ones.
+		gcRemoveStackBarriers(gp)
 
 	case _GCmarktermination:
+		if !work.markrootDone {
+			// This is a STW GC. There may be stale stack
+			// barriers from an earlier cycle since we
+			// never passed through mark phase.
+			gcRemoveStackBarriers(gp)
+		}
+
 		if int(gp.stkbarPos) == len(gp.stkbar) {
 			// gp hit all of the stack barriers (or there
 			// were none). Re-scan the whole stack.
@@ -683,8 +683,6 @@ func scanstack(gp *g) {
 			}
 		}
 
-		gcRemoveStackBarriers(gp)
-
 	default:
 		throw("scanstack in wrong phase")
 	}
@@ -722,9 +720,7 @@ func scanstack(gp *g) {
 	if gcphase == _GCmarktermination {
 		gcw.dispose()
 	}
-	if gcphase == _GCmark {
-		gcUnlockStackBarriers(gp)
-	}
+	gcUnlockStackBarriers(gp)
 	gp.gcscanvalid = true
 }
 
diff --git a/src/runtime/mstkbar.go b/src/runtime/mstkbar.go
index 016625ae92..f320c351d0 100644
--- a/src/runtime/mstkbar.go
+++ b/src/runtime/mstkbar.go
@@ -214,14 +214,15 @@ func gcInstallStackBarrier(gp *g, frame *stkframe) bool {
 }
 
 // gcRemoveStackBarriers removes all stack barriers installed in gp's stack.
+//
+// gp's stack barriers must be locked.
+//
 //go:nowritebarrier
 func gcRemoveStackBarriers(gp *g) {
 	if debugStackBarrier && gp.stkbarPos != 0 {
 		print("hit ", gp.stkbarPos, " stack barriers, goid=", gp.goid, "\n")
 	}
 
-	gcLockStackBarriers(gp)
-
 	// Remove stack barriers that we didn't hit.
 	for _, stkbar := range gp.stkbar[gp.stkbarPos:] {
 		gcRemoveStackBarrier(gp, stkbar)
@@ -231,8 +232,6 @@ func gcRemoveStackBarriers(gp *g) {
 	// adjust them.
 	gp.stkbarPos = 0
 	gp.stkbar = gp.stkbar[:0]
-
-	gcUnlockStackBarriers(gp)
 }
 
 // gcRemoveStackBarrier removes a single stack barrier. It is the