MVar swap

Hi, I took a look at the implementation of the MVar function swap and I am just woundering where the atomicy comes from. -- |Swap the contents of an 'MVar' for a new value. swapMVar :: MVar a -> a -> IO a swapMVar mvar new = block $ do old <- takeMVar mvar putMVar mvar new return old Is it really just the Exception block, that makes it not interruptable? Thanks! Georg