diff --git a/src/pkg/sync/once.go b/src/pkg/sync/once.go
index b6f5f5a8726..447b71dcb7a 100644
--- a/src/pkg/sync/once.go
+++ b/src/pkg/sync/once.go
@@ -4,10 +4,14 @@
 
 package sync
 
+import (
+	"sync/atomic"
+)
+
 // Once is an object that will perform exactly one action.
 type Once struct {
 	m    Mutex
-	done bool
+	done int32
 }
 
 // Do calls the function f if and only if the method is being called for the
@@ -26,10 +30,14 @@ type Once struct {
 // Do to be called, it will deadlock.
 //
 func (o *Once) Do(f func()) {
+	if atomic.AddInt32(&o.done, 0) == 1 {
+		return
+	}
+	// Slow-path.
 	o.m.Lock()
 	defer o.m.Unlock()
-	if !o.done {
-		o.done = true
+	if o.done == 0 {
 		f()
+		atomic.CompareAndSwapInt32(&o.done, 0, 1)
 	}
 }