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, 24 deletions

diff --git a/rts/HeapStackCheck.cmm b/rts/HeapStackCheck.cmm
index 001e5f6299..85fb1cbef6 100644
--- a/rts/HeapStackCheck.cmm
+++ b/rts/HeapStackCheck.cmm
@@ -230,30 +230,6 @@ stg_gc_prim_p_ll
 }
 
 /* -----------------------------------------------------------------------------
-   stg_enter_checkbh is just like stg_enter, except that we also call
-   checkBlockingQueues().  The point of this is that the GC can
-   replace an stg_marked_upd_frame with an stg_enter_checkbh if it
-   finds that the BLACKHOLE has already been updated by another
-   thread.  It would be unsafe to use stg_enter, because there might
-   be an orphaned BLOCKING_QUEUE now.
-   -------------------------------------------------------------------------- */
-
-/* The stg_enter_checkbh frame has the same shape as an update frame: */
-
-INFO_TABLE_RET ( stg_enter_checkbh, RET_SMALL,
-                 UPDATE_FRAME_FIELDS(W_,P_,info_ptr,ccs,p2,updatee))
-    return (P_ ret)
-{
-    foreign "C" checkBlockingQueues(MyCapability() "ptr",
-                                    CurrentTSO);
-
-    // we need to return updatee now.  Note that it might be a pointer
-    // to an indirection or a tagged value, we don't know which, so we
-    // need to ENTER() rather than return().
-    ENTER(updatee);
-}
-
-/* -----------------------------------------------------------------------------
    Info tables for returning values of various types.  These are used
    when we want to push a frame on the stack that will return a value
    to the frame underneath it.