runtime: make "write barriers are not allowed" comments more precise

Currently, various functions are marked with the comment

  // May run without a P, so write barriers are not allowed.

However, "running without a P" is ambiguous. We intended these to mean
that m.p may be nil (which is the condition checked by the write
barrier). The comment could also be taken to mean that a
stop-the-world may happen, which is not the case for these functions
because they run in situations where there is in fact a function on
the stack holding a P locally, it just isn't in m.p.

Change these comments to state precisely what we mean, that m.p may be
nil.

Change-Id: I4a4a1d26aebd455e5067540e13b9f96a7482146c
Reviewed-on: https://go-review.googlesource.com/8209Reviewed-by: Minux Ma <minux@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>

src/runtime/os1_darwin.go

@@ -71,7 +71,7 @@ func goenvs() {
 	}
 }
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, 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

src/runtime/os1_dragonfly.go

@@ -71,7 +71,7 @@ func futexwakeup(addr *uint32, cnt uint32) {
 
 func lwp_start(uintptr)
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {

src/runtime/os1_freebsd.go

@@ -67,7 +67,7 @@ func futexwakeup(addr *uint32, cnt uint32) {
 
 func thr_start()
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {

src/runtime/os1_linux.go

@@ -113,7 +113,7 @@ const (
 	_CLONE_NEWIPC         = 0x8000000
 )
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	/*

src/runtime/os1_nacl.go

@@ -67,7 +67,7 @@ func usleep(us uint32) {
 
 func mstart_nacl()
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	mp.tls[0] = uintptr(unsafe.Pointer(mp.g0))

src/runtime/os1_netbsd.go

@@ -89,7 +89,7 @@ func semawakeup(mp *m) {
 	}
 }
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {

src/runtime/os1_openbsd.go

@@ -98,7 +98,7 @@ func semawakeup(mp *m) {
 	}
 }
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {

src/runtime/os1_plan9.go

@@ -183,7 +183,7 @@ func exit(e int) {
 	exits(&status[0])
 }
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	if false {

src/runtime/os1_windows.go

@@ -295,7 +295,7 @@ func semacreate() uintptr {
 	return stdcall4(_CreateEventA, 0, 0, 0, 0)
 }
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, stk unsafe.Pointer) {
 	const _STACK_SIZE_PARAM_IS_A_RESERVATION = 0x00010000

src/runtime/os3_solaris.go

@@ -131,7 +131,7 @@ func osinit() {
 
 func tstart_sysvicall()
 
-// May run without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func newosproc(mp *m, _ unsafe.Pointer) {
 	var (

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 without a P, so write barriers are not allowed.
+// May run with m.p==nil, 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 without a P, so write barriers are not allowed.
+// May run with m.p==nil, so write barriers are not allowed.
 //go:nowritebarrier
 func startm(_p_ *p, spinning bool) {
 	lock(&sched.lock)