diff --git a/src/pkg/runtime/mem.go b/src/pkg/runtime/mem.go
index c3316d44c0..93d155a7f8 100644
--- a/src/pkg/runtime/mem.go
+++ b/src/pkg/runtime/mem.go
@@ -62,8 +62,13 @@ func init() {
 }
 
 // MemStats holds statistics about the memory system.
-// The statistics are only approximate, as they are not interlocked on update.
+// The statistics may be out of date, as the information is
+// updated lazily from per-thread caches.
+// Use UpdateMemStats to bring the statistics up to date.
 var MemStats MemStatsType
 
+// UpdateMemStats brings MemStats up to date.
+func UpdateMemStats()
+
 // GC runs a garbage collection.
 func GC()
diff --git a/src/pkg/runtime/mgc0.c b/src/pkg/runtime/mgc0.c
index bc373d8909..6325aadc67 100644
--- a/src/pkg/runtime/mgc0.c
+++ b/src/pkg/runtime/mgc0.c
@@ -663,6 +663,22 @@ runtime·gc(int32 force)
 		runtime·gc(1);
 }
 
+void
+runtime·UpdateMemStats(void)
+{
+	// Have to acquire gcsema to stop the world,
+	// because stoptheworld can only be used by
+	// one goroutine at a time, and there might be
+	// a pending garbage collection already calling it.
+	runtime·semacquire(&gcsema);
+	m->gcing = 1;
+	runtime·stoptheworld();
+	cachestats();
+	m->gcing = 0;
+	runtime·semrelease(&gcsema);
+	runtime·starttheworld();
+}
+
 static void
 runfinq(void)
 {
diff --git a/test/gc2.go b/test/gc2.go
index c5c6cbe4bb..c54d807df7 100644
--- a/test/gc2.go
+++ b/test/gc2.go
@@ -32,7 +32,8 @@ func main() {
 			}
 		}
 	}
-	
+
+	runtime.UpdateMemStats()
 	obj := runtime.MemStats.HeapObjects - st.HeapObjects
 	if obj > N/5 {
 		fmt.Println("too many objects left:", obj)
diff --git a/test/malloc1.go b/test/malloc1.go
index 146976467b..61f1797c75 100644
--- a/test/malloc1.go
+++ b/test/malloc1.go
@@ -18,6 +18,7 @@ var chatty = flag.Bool("v", false, "chatty")
 
 func main() {
 	runtime.Free(runtime.Alloc(1))
+	runtime.UpdateMemStats()
 	if *chatty {
 		fmt.Printf("%+v %v\n", runtime.MemStats, uint64(0))
 	}
diff --git a/test/mallocrand.go b/test/mallocrand.go
index e6b422e224..f014b441b2 100644
--- a/test/mallocrand.go
+++ b/test/mallocrand.go
@@ -21,6 +21,7 @@ var footprint uint64
 var allocated uint64
 
 func bigger() {
+	runtime.UpdateMemStats()
 	if f := runtime.MemStats.Sys; footprint < f {
 		footprint = f
 		if *chatty {
diff --git a/test/mallocrep.go b/test/mallocrep.go
index 43233b7b74..9f47e52e2b 100644
--- a/test/mallocrep.go
+++ b/test/mallocrep.go
@@ -18,6 +18,7 @@ var chatty = flag.Bool("v", false, "chatty")
 var oldsys uint64
 
 func bigger() {
+	runtime.UpdateMemStats()
 	if st := runtime.MemStats; oldsys < st.Sys {
 		oldsys = st.Sys
 		if *chatty {
@@ -31,7 +32,7 @@ func bigger() {
 }
 
 func main() {
-	runtime.GC()		   // clean up garbage from init
+	runtime.GC()               // clean up garbage from init
 	runtime.MemProfileRate = 0 // disable profiler
 	runtime.MemStats.Alloc = 0 // ignore stacks
 	flag.Parse()
@@ -45,9 +46,10 @@ func main() {
 				panic("fail")
 			}
 			b := runtime.Alloc(uintptr(j))
+			runtime.UpdateMemStats()
 			during := runtime.MemStats.Alloc
 			runtime.Free(b)
-			runtime.GC()
+			runtime.UpdateMemStats()
 			if a := runtime.MemStats.Alloc; a != 0 {
 				println("allocated ", j, ": wrong stats: during=", during, " after=", a, " (want 0)")
 				panic("fail")
diff --git a/test/mallocrep1.go b/test/mallocrep1.go
index 079ae94226..0b1479900e 100644
--- a/test/mallocrep1.go
+++ b/test/mallocrep1.go
@@ -42,6 +42,7 @@ func AllocAndFree(size, count int) {
 	if *chatty {
 		fmt.Printf("size=%d count=%d ...\n", size, count)
 	}
+	runtime.UpdateMemStats()
 	n1 := stats.Alloc
 	for i := 0; i < count; i++ {
 		b[i] = runtime.Alloc(uintptr(size))
@@ -50,17 +51,18 @@ func AllocAndFree(size, count int) {
 			println("lookup failed: got", base, n, "for", b[i])
 			panic("fail")
 		}
-		if runtime.MemStats.Sys > 1e9 {
+		runtime.UpdateMemStats()
+		if stats.Sys > 1e9 {
 			println("too much memory allocated")
 			panic("fail")
 		}
 	}
+	runtime.UpdateMemStats()
 	n2 := stats.Alloc
 	if *chatty {
 		fmt.Printf("size=%d count=%d stats=%+v\n", size, count, *stats)
 	}
 	n3 := stats.Alloc
-	runtime.GC()
 	for j := 0; j < count; j++ {
 		i := j
 		if *reverse {
@@ -73,7 +75,7 @@ func AllocAndFree(size, count int) {
 			panic("fail")
 		}
 		runtime.Free(b[i])
-		runtime.GC()
+		runtime.UpdateMemStats()
 		if stats.Alloc != uint64(alloc-n) {
 			println("free alloc got", stats.Alloc, "expected", alloc-n, "after free of", n)
 			panic("fail")
@@ -83,6 +85,7 @@ func AllocAndFree(size, count int) {
 			panic("fail")
 		}
 	}
+	runtime.UpdateMemStats()
 	n4 := stats.Alloc
 
 	if *chatty {