Question about withMVar

The documentation of withMVar (in Control.Concurrent.MVar module of base
package) says:

{-|  'withMVar' is an exception-safe wrapper for operating on the
contents  of an 'MVar'.  This operation is exception-safe: it will
replace the  original contents of the 'MVar' if an exception is raised
(see  "Control.Exception").  However, it is only atomic if there are
no  other producers for this 'MVar'.-}withMVar :: MVar a -> (a -> IO
b) -> IO bwithMVar m io =  mask $ \restore -> do    a <- takeMVar m
b <- restore (io a) `onException` putMVar m a    putMVar m a    return
b

Can someone shed some light on what is meant by the statement -
"However, it is only atomic if there are no other producers for this 'MVar'."?

I hope this is the right mailing list for this question.

Thanks,
Harendra

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