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, 0 deletions

diff --git a/rts/StgStartup.cmm b/rts/StgStartup.cmm
index a650e5c367..0cd18628e2 100644
--- a/rts/StgStartup.cmm
+++ b/rts/StgStartup.cmm
@@ -127,6 +127,9 @@ stg_returnToSched /* no args: explicit stack layout */
 // current thread.  This is used for switching from compiled execution to the
 // interpreter, where calling threadPaused() on every switch would be too
 // expensive.
+//
+// See Note [avoiding threadPaused] in Interpreter.c
+//
 stg_returnToSchedNotPaused /* no args: explicit stack layout */
 {
   SAVE_THREAD_STATE();