[Haskell-cafe] MVars and locks

[Tom Ellis]

In the following section of the book "Parallel and Concurrent Programming in
Haskell"

    http://chimera.labs.oreilly.com/books/1230000000929/ch07.html#sec_conc-phonebook

Simon Marlow explains that MVars can be used to implement something like
locks on shared functional state: "To acquire the lock, we take the MVar,
whereas, to update the variable and release the lock, we put the MVar."

Am I right in thinking this only holds if we are careful to ensure both
those operations happen in the given order?

For example, if I take the MVar (lock) and I am about to put something back
in (unlock) but someone else puts something in before I do, then the lock
system becomes broken, doesn't it?  So if we want to be sure we have a
robust lock system we have to wrap up MVars in another abstraction?

Tom