Previously, the main mount loop was unlocking the thread and then immediately

re-locking it.  In some threads implementations (Linux 2.6), this causes the
mount thread to hog the lock, which causes SetMountThreadState to hang.  Move
the "check and delay" part of the mount thread outside of the lock, so other
threads have opportunity to get the lock.

stepmania/src/arch/MemoryCard/MemoryCardDriverThreaded_Linux.cpp

@@ -187,32 +187,29 @@ bool UsbStorageDevicesChanged()
   return bChanged;
 }
 
-void MemoryCardDriverThreaded_Linux::MountThreadDoOneUpdate()
+/* Return true if MountThreadDoOneUpdate should be called. */
+bool MemoryCardDriverThreaded_Linux::MountThreadWaitForUpdate()
 {
-	bool bNeedToDoAnyMounts = false;
+	/* Check if any devices need a write test. */
 	for( unsigned i=0; i<m_vDevicesLastSeen.size(); i++ )
 	{
 		UsbStorageDevice &d = m_vDevicesLastSeen[i];
 		if( d.bNeedsWriteTest )
-		{
-			bNeedToDoAnyMounts = true;
-			break;
-		}
+			return true;
 	}
-	
-	if( bNeedToDoAnyMounts )
-	{
-		// fall through
-	}
-	else
-	{
-	  if( !UsbStorageDevicesChanged() )
-	    {
-	      usleep(1000*300);  // 300 ms
-	      return;
-	    }
-	}
-	
+
+	/* Nothing needs a write test.  If no devices have changed, either,
+	 * delay. */
+	if( UsbStorageDevicesChanged() )
+		return true;
+
+	/* Nothing to do.  Delay, so we don't busy loop. */
+	usleep(1000*300);  // 300 ms
+	return false;
+}
+
+void MemoryCardDriverThreaded_Linux::MountThreadDoOneUpdate()
+{
 	// TRICKY: We're waiting for a change in the USB device list, but 
 	// the usb-storage descriptors take a bit longer to update.  It's more convenient to wait
 	// on the USB device list because the usb-storage descriptors are separate files per