diff --git a/src/runtime/os1_windows.go b/src/runtime/os1_windows.go
index 4ae1a84..609e430 100644
--- a/src/runtime/os1_windows.go
+++ b/src/runtime/os1_windows.go
@@ -282,6 +282,8 @@
 	return stdcall4(_CreateEventA, 0, 0, 0, 0)
 }
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	const _STACK_SIZE_PARAM_IS_A_RESERVATION = 0x00010000
 	thandle := stdcall6(_CreateThread, 0, 0x20000,