Fix a lost-wakeup bug in BLACKHOLE handling (#13751)

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

1 files changed, 0 insertions, 2 deletions

diff --git a/includes/stg/MiscClosures.h b/includes/stg/MiscClosures.h
index 4e8fe68bd6..76cfbd6c8c 100644
--- a/includes/stg/MiscClosures.h
+++ b/includes/stg/MiscClosures.h
@@ -275,8 +275,6 @@ RTS_FUN_DECL(stg_PAP_apply);
 
 RTS_FUN_DECL(stg_gc_noregs);
 
-RTS_RET(stg_enter_checkbh);
-
 RTS_RET(stg_ret_v);
 RTS_RET(stg_ret_p);
 RTS_RET(stg_ret_n);