How does lazy evaluation interplay with MVars?

Yes, it's real. You get around it by avoiding all the base functions that are implemented using unsafeInterleaveIO. I don't have a complete list, but that's at least readFile, getContents, hGetContents. IO actions that don't do lazy IO -- like hGet or hGetLine -- are fine.

If you must use lazy IO, then fully evaluate its results in an IO action inside the critical section, e.g. by combining rnf and evaluate.

Some other commentary on related things, but that aren't directly answers to this question:

• Laziness and lazy IO are really separate concepts. They happen to share a name because humans are lazy at naming. Most IO actions do not involve lazy IO and do not run into this problem.

• There is a related problem about stuffing unevaluated pure computations into your MVar and accidentally evaluating it on a different thread than you were expecting, but if you avoid lazy IO then evaluating on the wrong thread is merely a performance bug rather than an actual semantics bug.