runtime: ensure _Grunning Gs have a valid g.m and g.m.p

We already claim on the documentation for _Grunning that this is case,
but execute transitions to _Grunning before assigning g.m. Fix this
and make the documentation even more explicit.

For #10958, #24543, but also a good cleanup.

Change-Id: I1eb0108e7762f55cfb0282aca624af1c0a15fe56
Reviewed-on: https://go-review.googlesource.com/c/go/+/201440
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/runtime/proc.go

@@ -2177,6 +2177,10 @@ func gcstopm() {
 func execute(gp *g, inheritTime bool) {
 	_g_ := getg()
 
+	// Assign gp.m before entering _Grunning so running Gs have an
+	// M.
+	_g_.m.curg = gp
+	gp.m = _g_.m
 	casgstatus(gp, _Grunnable, _Grunning)
 	gp.waitsince = 0
 	gp.preempt = false
@@ -2184,8 +2188,6 @@ func execute(gp *g, inheritTime bool) {
 	if !inheritTime {
 		_g_.m.p.ptr().schedtick++
 	}
-	_g_.m.curg = gp
-	gp.m = _g_.m
 
 	// Check whether the profiler needs to be turned on or off.
 	hz := sched.profilehz

src/runtime/runtime2.go

@@ -40,7 +40,7 @@ const (
 
 	// _Grunning means this goroutine may execute user code. The
 	// stack is owned by this goroutine. It is not on a run queue.
-	// It is assigned an M and a P.
+	// It is assigned an M and a P (g.m and g.m.p are valid).
 	_Grunning // 2
 
 	// _Gsyscall means this goroutine is executing a system call.