add memory fences to try to avoid a conceptual memory ordering issue between capture & the butler

libs/ardour/butler.cc

@@ -213,6 +213,9 @@ Butler::thread_work ()
 		Temporal::TempoMap::fetch ();
 
 	restart:
+		/* Ensure that no reads migrate before this fence */
+		std::atomic_thread_fence (std::memory_order_acquire);
+
 		DEBUG_TRACE (DEBUG::Butler, "at restart for disk work\n");
 		disk_work_outstanding = false;
 

libs/ardour/disk_writer.cc

@@ -737,6 +737,10 @@ DiskWriter::run (BufferSet& bufs, samplepos_t start_sample, samplepos_t end_samp
 		_need_butler = true;
 	}
 
+	/* Ensure that anything written during run() is visible in other threads */
+
+	std::atomic_thread_fence (std::memory_order_release);
+
 	// DEBUG_TRACE (DEBUG::Butler, string_compose ("%1 writer run, needs butler = %2\n", name(), _need_butler));
 }