diff --git a/src/runtime/os1_darwin.go b/src/runtime/os1_darwin.go
index 6a60314494..f6eb557b81 100644
--- a/src/runtime/os1_darwin.go
+++ b/src/runtime/os1_darwin.go
@@ -71,6 +71,8 @@ func goenvs() {
 	}
 }
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	mp.tls[0] = uintptr(mp.id) // so 386 asm can find it
 	if false {
diff --git a/src/runtime/os1_dragonfly.go b/src/runtime/os1_dragonfly.go
index c94b1411b0..33d87ebdd9 100644
--- a/src/runtime/os1_dragonfly.go
+++ b/src/runtime/os1_dragonfly.go
@@ -71,6 +71,8 @@ func futexwakeup(addr *uint32, cnt uint32) {
 
 func lwp_start(uintptr)
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {
 		print("newosproc stk=", stk, " m=", mp, " g=", mp.g0, " lwp_start=", funcPC(lwp_start), " id=", mp.id, "/", mp.tls[0], " ostk=", &mp, "\n")
diff --git a/src/runtime/os1_freebsd.go b/src/runtime/os1_freebsd.go
index ae9f78c27b..10c72d1d7e 100644
--- a/src/runtime/os1_freebsd.go
+++ b/src/runtime/os1_freebsd.go
@@ -67,6 +67,8 @@ func futexwakeup(addr *uint32, cnt uint32) {
 
 func thr_start()
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {
 		print("newosproc stk=", stk, " m=", mp, " g=", mp.g0, " thr_start=", funcPC(thr_start), " id=", mp.id, "/", mp.tls[0], " ostk=", &mp, "\n")
diff --git a/src/runtime/os1_linux.go b/src/runtime/os1_linux.go
index 44da57ab0b..190206dcb6 100644
--- a/src/runtime/os1_linux.go
+++ b/src/runtime/os1_linux.go
@@ -113,6 +113,8 @@ const (
 	_CLONE_NEWIPC         = 0x8000000
 )
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	/*
 	 * note: strace gets confused if we use CLONE_PTRACE here.
diff --git a/src/runtime/os1_nacl.go b/src/runtime/os1_nacl.go
index b3759c1408..238de5b441 100644
--- a/src/runtime/os1_nacl.go
+++ b/src/runtime/os1_nacl.go
@@ -67,6 +67,7 @@ func usleep(us uint32) {
 
 func mstart_nacl()
 
+// May run without a P, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	mp.tls[0] = uintptr(unsafe.Pointer(mp.g0))
diff --git a/src/runtime/os1_netbsd.go b/src/runtime/os1_netbsd.go
index f2e6ef682e..9a401550ba 100644
--- a/src/runtime/os1_netbsd.go
+++ b/src/runtime/os1_netbsd.go
@@ -89,6 +89,8 @@ func semawakeup(mp *m) {
 	}
 }
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {
 		print("newosproc stk=", stk, " m=", mp, " g=", mp.g0, " id=", mp.id, "/", int32(mp.tls[0]), " ostk=", &mp, "\n")
diff --git a/src/runtime/os1_openbsd.go b/src/runtime/os1_openbsd.go
index 92a19fe31b..1f5ac4aa31 100644
--- a/src/runtime/os1_openbsd.go
+++ b/src/runtime/os1_openbsd.go
@@ -98,6 +98,8 @@ func semawakeup(mp *m) {
 	}
 }
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {
 		print("newosproc stk=", stk, " m=", mp, " g=", mp.g0, " id=", mp.id, "/", int32(mp.tls[0]), " ostk=", &mp, "\n")
diff --git a/src/runtime/os1_plan9.go b/src/runtime/os1_plan9.go
index bba1f17e32..30621ad561 100644
--- a/src/runtime/os1_plan9.go
+++ b/src/runtime/os1_plan9.go
@@ -183,6 +183,8 @@ func exit(e int) {
 	exits(&status[0])
 }
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {
 		print("newosproc mp=", mp, " ostk=", &mp, "\n")
diff --git a/src/runtime/os1_windows.go b/src/runtime/os1_windows.go
index 4ae1a8411b..609e430711 100644
--- a/src/runtime/os1_windows.go
+++ b/src/runtime/os1_windows.go
@@ -282,6 +282,8 @@ func semacreate() uintptr {
 	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,
diff --git a/src/runtime/os3_solaris.go b/src/runtime/os3_solaris.go
index 8c65567b83..ddec87e7e2 100644
--- a/src/runtime/os3_solaris.go
+++ b/src/runtime/os3_solaris.go
@@ -131,6 +131,8 @@ func osinit() {
 
 func tstart_sysvicall()
 
+// May run without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func newosproc(mp *m, _ unsafe.Pointer) {
 	var (
 		attr pthreadattr
diff --git a/src/runtime/proc1.go b/src/runtime/proc1.go
index 4459802b15..690a978919 100644
--- a/src/runtime/proc1.go
+++ b/src/runtime/proc1.go
@@ -973,7 +973,7 @@ func unlockextra(mp *m) {
 
 // Create a new m.  It will start off with a call to fn, or else the scheduler.
 // fn needs to be static and not a heap allocated closure.
-// May run during STW, so write barriers are not allowed.
+// May run without a P, so write barriers are not allowed.
 //go:nowritebarrier
 func newm(fn func(), _p_ *p) {
 	mp := allocm(_p_)
@@ -1035,7 +1035,7 @@ func mspinning() {
 
 // Schedules some M to run the p (creates an M if necessary).
 // If p==nil, tries to get an idle P, if no idle P's does nothing.
-// May run during STW, so write barriers are not allowed.
+// May run without a P, so write barriers are not allowed.
 //go:nowritebarrier
 func startm(_p_ *p, spinning bool) {
 	lock(&sched.lock)
@@ -1072,6 +1072,8 @@ func startm(_p_ *p, spinning bool) {
 }
 
 // Hands off P from syscall or locked M.
+// Always runs without a P, so write barriers are not allowed.
+//go:nowritebarrier
 func handoffp(_p_ *p) {
 	// if it has local work, start it straight away
 	if _p_.runqhead != _p_.runqtail || sched.runqsize != 0 {