runtime: fix data race in findfunc()

The data race can lead to reads of partially
initialized concurrently mutated symbol data.
The change also adds a simple sanity test
for Caller() and FuncForPC().

R=rsc
CC=golang-dev
https://golang.org/cl/4817058

src/pkg/runtime/386/asm.s

@@ -354,6 +354,12 @@ TEXT runtime·atomicstorep(SB), 7, $0
 	XCHGL	AX, 0(BX)
 	RET
 
+TEXT runtime·atomicstore(SB), 7, $0
+	MOVL	4(SP), BX
+	MOVL	8(SP), AX
+	XCHGL	AX, 0(BX)
+	RET
+
 // void jmpdefer(fn, sp);
 // called from deferreturn.
 // 1. pop the caller

src/pkg/runtime/amd64/asm.s

@@ -398,6 +398,12 @@ TEXT runtime·atomicstorep(SB), 7, $0
 	XCHGQ	AX, 0(BX)
 	RET
 
+TEXT runtime·atomicstore(SB), 7, $0
+	MOVQ	8(SP), BX
+	MOVL	16(SP), AX
+	XCHGL	AX, 0(BX)
+	RET
+
 // void jmpdefer(fn, sp);
 // called from deferreturn.
 // 1. pop the caller

src/pkg/runtime/arm/atomic.c

@@ -68,3 +68,16 @@ runtime·atomicstorep(void* volatile* addr, void* v)
 			return;
 	}
 }
+
+#pragma textflag 7
+void
+runtime·atomicstore(uint32 volatile* addr, uint32 v)
+{
+	uint32 old;
+	
+	for(;;) {
+		old = *addr;
+		if(runtime·cas(addr, old, v))
+			return;
+	}
+}
\ No newline at end of file

src/pkg/runtime/runtime.h

@@ -433,6 +433,7 @@ bool	runtime·casp(void**, void*, void*);
 uint32	runtime·xadd(uint32 volatile*, int32);
 uint32	runtime·xchg(uint32 volatile*, uint32);
 uint32	runtime·atomicload(uint32 volatile*);
+void	runtime·atomicstore(uint32 volatile*, uint32);
 void*	runtime·atomicloadp(void* volatile*);
 void	runtime·atomicstorep(void* volatile*, void*);
 void	runtime·jmpdefer(byte*, void*);

src/pkg/runtime/symtab.c

@@ -78,6 +78,7 @@ static int32 nfunc;
 static byte **fname;
 static int32 nfname;
 
+static uint32 funcinit;
 static Lock funclock;
 
 static void
@@ -427,10 +428,12 @@ runtime·findfunc(uintptr addr)
 	// (Before enabling the signal handler,
 	// SetCPUProfileRate calls findfunc to trigger
 	// the initialization outside the handler.)
-	if(runtime·atomicloadp(&func) == nil) {
+	if(runtime·atomicload(&funcinit) == 0) {
 		runtime·lock(&funclock);
-		if(func == nil)
+		if(funcinit == 0) {
 			buildfuncs();
+			runtime·atomicstore(&funcinit, 1);
+		}
 		runtime·unlock(&funclock);
 	}
 

src/pkg/runtime/symtab_test.go

+// Copyright 2009 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package runtime_test
+
+import (
+	"runtime"
+	"strings"
+	"testing"
+)
+
+func TestCaller(t *testing.T) {
+	procs := runtime.GOMAXPROCS(-1)
+	c := make(chan bool, procs)
+	for p := 0; p < procs; p++ {
+		go func() {
+			for i := 0; i < 1000; i++ {
+				testCallerFoo(t)
+			}
+			c <- true
+		}()
+		defer func() {
+			<-c
+		}()
+	}
+}
+
+func testCallerFoo(t *testing.T) {
+	testCallerBar(t)
+}
+
+func testCallerBar(t *testing.T) {
+	for i := 0; i < 2; i++ {
+		pc, file, line, ok := runtime.Caller(i)
+		f := runtime.FuncForPC(pc)
+		if !ok ||
+			!strings.HasSuffix(file, "symtab_test.go") ||
+			(i == 0 && !strings.HasSuffix(f.Name(), "testCallerBar")) ||
+			(i == 1 && !strings.HasSuffix(f.Name(), "testCallerFoo")) ||
+			line < 5 || line > 1000 ||
+			f.Entry() >= pc {
+			t.Errorf("incorrect symbol info %d: %t %d %d %s %s %d",
+				i, ok, f.Entry(), pc, f.Name(), file, line)
+		}
+	}
+}