From 94dc963b558b3d37906af53eca45c5ae807a9e84 Mon Sep 17 00:00:00 2001
From: Dmitriy Vyukov <dvyukov@google.com>
Date: Sat, 15 Jun 2013 16:07:06 +0400
Subject: [PATCH] runtime: fix race condition between GC and setGCPercent If
 first GC runs concurrently with setGCPercent, it can overwrite gcpercent
 value with default.

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/10242047
---
 src/pkg/runtime/mgc0.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
index a55ee49c777..4be0eeb0dbd 100644
--- a/src/pkg/runtime/mgc0.c
+++ b/src/pkg/runtime/mgc0.c
@@ -1974,7 +1974,10 @@ runtime·gc(int32 force)
 		return;
 
 	if(gcpercent == GcpercentUnknown) {	// first time through
-		gcpercent = readgogc();
+		runtime·lock(&runtime·mheap);
+		if(gcpercent == GcpercentUnknown)
+			gcpercent = readgogc();
+		runtime·unlock(&runtime·mheap);
 
 		p = runtime·getenv("GOGCTRACE");
 		if(p != nil)