diff --git a/src/runtime/os1_windows.go b/src/runtime/os1_windows.go
index 561d3ce4575..4ae1a8411b6 100644
--- a/src/runtime/os1_windows.go
+++ b/src/runtime/os1_windows.go
@@ -359,6 +359,8 @@ func nanotime() int64 {
 }
 
 // Calling stdcall on os stack.
+// May run during STW, so write barriers are not allowed.
+//go:nowritebarrier
 //go:nosplit
 func stdcall(fn stdFunction) uintptr {
 	gp := getg()
@@ -367,7 +369,8 @@ func stdcall(fn stdFunction) uintptr {
 
 	if mp.profilehz != 0 {
 		// leave pc/sp for cpu profiler
-		mp.libcallg = gp
+		// gp is on allg, so this WB can be eliminated.
+		setGNoWriteBarrier(&mp.libcallg, gp)
 		mp.libcallpc = getcallerpc(unsafe.Pointer(&fn))
 		// sp must be the last, because once async cpu profiler finds
 		// all three values to be non-zero, it will use them