runtime: adopt race detector for runtime written in Go

Ignore memory access on g0/gsignal.
See the issue for context and explanation.
Fixes #8627.

LGTM=khr
R=golang-codereviews, mdempsky, khr
CC=golang-codereviews, rsc
https://golang.org/cl/137070043

src/pkg/runtime/cpuprof.go

@@ -239,9 +239,7 @@ Assoc:
 	// Reuse the newly evicted entry.
 	e.depth = uintptr(len(pc))
 	e.count = 1
-	for i := range pc {
-		e.stack[i] = pc[i]
-	}
+	copy(e.stack[:], pc)
 }
 
 // evict copies the given entry's data into the log, so that
@@ -266,10 +264,8 @@ func (p *cpuProfile) evict(e *cpuprofEntry) bool {
 	q++
 	log[q] = d
 	q++
-	for i := uintptr(0); i < d; i++ {
-		log[q] = e.stack[i]
-		q++
-	}
+	copy(log[q:], e.stack[:d])
+	q += d
 	p.nlog = q
 	e.count = 0
 	return true

src/pkg/runtime/proc.c

@@ -214,7 +214,11 @@ void
 runtime·main(void)
 {
 	Defer d;
-	
+
+	// Racectx of m0->g0 is used only as the parent of the main goroutine.
+	// It must not be used for anything else.
+	g->m->g0->racectx = 0;
+
 	// Max stack size is 1 GB on 64-bit, 250 MB on 32-bit.
 	// Using decimal instead of binary GB and MB because
 	// they look nicer in the stack overflow failure message.
@@ -1166,8 +1170,6 @@ newm(void(*fn)(void), P *p)
 	mp = runtime·allocm(p);
 	mp->nextp = p;
 	mp->mstartfn = fn;
-	if(raceenabled)
-		mp->g0->racectx = runtime·racegostart(newm);
 
 	if(runtime·iscgo) {
 		CgoThreadStart ts;

src/pkg/runtime/race.c

@@ -153,6 +153,11 @@ runtime·racegoend(void)
 void
 runtime·racewriterangepc(void *addr, uintptr sz, void *callpc, void *pc)
 {
+	if(g != g->m->curg) {
+		// The call is coming from manual instrumentation of Go code running on g0/gsignal.
+		// Not interesting.
+		return;
+	}
 	if(callpc != nil)
 		runtime·racefuncenter(callpc);
 	runtime·racewriterangepc1(addr, sz, pc);
@@ -163,6 +168,11 @@ runtime·racewriterangepc(void *addr, uintptr sz, void *callpc, void *pc)
 void
 runtime·racereadrangepc(void *addr, uintptr sz, void *callpc, void *pc)
 {
+	if(g != g->m->curg) {
+		// The call is coming from manual instrumentation of Go code running on g0/gsignal.
+		// Not interesting.
+		return;
+	}
 	if(callpc != nil)
 		runtime·racefuncenter(callpc);
 	runtime·racereadrangepc1(addr, sz, pc);