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David Feuer authored
When `readMVar` was implemented using `takeMVar` and `putMVar`, we needed to use `modifyMVarMasked` in `readChan` just in case the `readMVar` was interrupted between taking and putting. Now that `readMVar` uses an atomic primop, this is impossible, so we can safely unmask `readMVar`. Reviewers: hvr, bgamari, simonmar Reviewed By: simonmar Subscribers: rwbarton, thomie, carter Differential Revision: https://phabricator.haskell.org/D4738
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