I don't think this is needed, but it's harmless and saves me from worrying.

src/cpp/ripple/ValidationCollection.cpp

@@ -330,3 +330,11 @@ void ValidationCollection::doWrite()
 	}
 	mWriting = false;
 }
+
+void ValidationCollection::sweep()
+{
+	boost::mutex::scoped_lock sl(mValidationLock);
+	mValidations.sweep();
+}
+
+// vim:ts=4

src/cpp/ripple/ValidationCollection.h

@@ -50,7 +50,7 @@ public:
 
 	void tune(int size, int age);
 	void flush();
-	void sweep() { mValidations.sweep(); }
+	void sweep();
 };
 
 #endif