runtime: two proc bug fixes

1. Fix bug in GOMAXPROCS when trying to cut number of procs
Race could happen on any system but was
manifesting only on Xen hosted Linux.

2. Fix recover on ARM, where FP != caller SP.

R=r
CC=golang-dev
https://golang.org/cl/880043

src/pkg/runtime/proc.c

@@ -352,6 +352,10 @@ stoptheworld(void)
 	gcwaiting = 1;
 	sched.mcpumax = 1;
 	while(sched.mcpu > 1) {
+		// It would be unsafe for multiple threads to be using
+		// the stopped note at once, but there is only
+		// ever one thread doing garbage collection,
+		// so this is okay.
 		noteclear(&sched.stopped);
 		sched.waitstop = 1;
 		unlock(&sched);
@@ -989,6 +993,8 @@ void
 	Stktop *top, *oldtop;
 	Panic *p;
 
+	fp = getcallersp(fp);
+
 	// Must be a panic going on.
 	if((p = g->panic) == nil || p->recovered)
 		goto nomatch;
@@ -1113,13 +1119,14 @@ void
 	lock(&sched);
 	sched.gomaxprocs = n;
 	sched.mcpumax = n;
-	// handle fewer procs
-	while(sched.mcpu > sched.mcpumax) {
-		noteclear(&sched.stopped);
-		sched.waitstop = 1;
+	// handle fewer procs?
+	if(sched.mcpu > sched.mcpumax) {
 		unlock(&sched);
-		notesleep(&sched.stopped);
-		lock(&sched);
+		// just give up the cpu.
+		// we'll only get rescheduled once the
+		// number has come down.
+		gosched();
+		return;
 	}
 	// handle more procs
 	matchmg();

test/recover2.go

@@ -15,6 +15,7 @@ package main
 import (
 	"os"
 	"strings"
+	"syscall"
 )
 
 var x = make([]byte, 10)
@@ -80,6 +81,10 @@ func test6() {
 }
 
 func test7() {
+	if syscall.ARCH == "arm" {
+		// ARM doesn't have floating point yet
+		return
+	}
 	defer mustRecover("complex divide by zero")
 	var x, y complex
 	println(x / y)