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, 3 insertions, 1 deletions

diff --git a/rts/Messages.c b/rts/Messages.c
index 0508a20d20..8fab314bc4 100644
--- a/rts/Messages.c
+++ b/rts/Messages.c
@@ -289,7 +289,9 @@ loop:
             recordClosureMutated(cap,(StgClosure*)bq);
         }
 
-        debugTraceCap(DEBUG_sched, cap, "thread %d blocked on thread %d",
+        debugTraceCap(DEBUG_sched, cap,
+                      "thread %d blocked on existing BLOCKING_QUEUE "
+                      "owned by thread %d",
                       (W_)msg->tso->id, (W_)owner->id);
 
         // See above, #3838