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Simon Marlow authored
Summary: The problem occurred when * Threads A & B evaluate the same thunk * Thread A context-switches, so the thunk gets blackholed * Thread C enters the blackhole, creates a BLOCKING_QUEUE attached to the blackhole and thread A's `tso->bq` queue * Thread B updates the blackhole with a value, overwriting the BLOCKING_QUEUE * We GC, replacing A's update frame with stg_enter_checkbh * Throw an exception in A, which ignores the stg_enter_checkbh frame Now we have C blocked on A's tso->bq queue, but we forgot to check the queue because the stg_enter_checkbh frame has been thrown away by the exception. The solution and alternative designs are discussed in Note [upd-black-hole]. This also exposed a bug in the interpreter, whereby we were sometimes context-switching without calling `threadPaused()`. I've fixed this and added some Notes. Test Plan: * `cd testsuite/tests/concurrent && make slow` * validate Reviewers: niteria, bgamari, austin, erikd Reviewed By: erikd Subscribers: rwbarton, thomie GHC Trac Issues: #13751 Differential Revision: https://phabricator.haskell.org/D3630
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