mask in waitQSem

Hello, I was reviewing some exception handling code in base library, and I found this line: https://phabricator.haskell.org/diffusion/GHC/browse/master/libraries/base/C... Here mask is used, but I looks completely useless for me. waitQSem itself should be called with async exceptions masked, otherwise there is no way to prevent resource leak. Do anyone know why mask is used here? I wonder whether an author of the code tried to do something different, so there actually can be a bug hidden here. Probably uninterruptibleMask_ should be used here? (I don't think so though.) Thanks, Yuras