[dev.garbage] runtime: concurrent mark fixes

Add missing write barrier when initializing state
for newly created goroutine. Add write barrier for
same slot when preempting a goroutine.

Disable write barrier during goroutine death,
because dopanic does pointer writes.

With concurrent mark enabled (not in this CL), all.bash passed once.
The second time, TestGoexitCrash-2 failed.

LGTM=rlh
R=rlh
CC=golang-codereviews
https://golang.org/cl/167610043

src/runtime/mgc0.c

@@ -1094,8 +1094,7 @@ shade(byte *b)
 void
 runtime·gcmarkwb_m()
 {
-	byte **slot, *ptr;
-	slot = (byte**)g->m->scalararg[0];
+	byte *ptr;
 	ptr = (byte*)g->m->scalararg[1];
 
 	switch(runtime·gcphase) {

src/runtime/mgc0.go

@@ -109,7 +109,7 @@ func writebarrierptr_nostore(dst *uintptr, src uintptr) {
 	}
 
 	mp := acquirem()
-	if mp.inwb {
+	if mp.inwb || mp.dying > 0 {
 		releasem(mp)
 		return
 	}

src/runtime/runtime.h

@@ -1121,6 +1121,8 @@ void	runtime·osyield(void);
 void	runtime·lockOSThread(void);
 void	runtime·unlockOSThread(void);
 
+void	runtime·writebarrierptr_nostore(void*, void*);
+
 bool	runtime·showframe(Func*, G*);
 void	runtime·printcreatedby(G*);
 

src/runtime/stack.c

@@ -706,6 +706,14 @@ runtime·newstack(void)
 		runtime·printf("runtime: split stack overflow: %p < %p\n", sp, gp->stack.lo);
 		runtime·throw("runtime: split stack overflow");
 	}
+	
+	if(gp->sched.ctxt != nil) {
+		// morestack wrote sched.ctxt on its way in here,
+		// without a write barrier. Run the write barrier now.
+		// It is not possible to be preempted between then
+		// and now, so it's okay.
+		runtime·writebarrierptr_nostore(&gp->sched.ctxt, gp->sched.ctxt);
+	}
 
 	if(gp->stackguard0 == (uintptr)StackPreempt) {
 		if(gp == g->m->g0)

src/runtime/sys_x86.c

@@ -20,6 +20,7 @@ runtime·gostartcall(Gobuf *gobuf, void (*fn)(void), void *ctxt)
 	gobuf->sp = (uintptr)sp;
 	gobuf->pc = (uintptr)fn;
 	gobuf->ctxt = ctxt;
+	runtime·writebarrierptr_nostore(&gobuf->ctxt, ctxt);
 }
 
 // Called to rewind context saved during morestack back to beginning of function.