[GHC] #15449: Nondeterministic Failure on aarch64 with -jn, n > 1

[GHC]

#15449: Nondeterministic Failure on aarch64 with -jn, n > 1
-------------------------------------+-------------------------------------
        Reporter:  tmobile           |                Owner:  tmobile
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.10.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Linux             |         Architecture:  aarch64
 Type of failure:  Compile-time      |            Test Case:
  crash or panic                     |
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by tmobile):

 I've spent a bunch of time trying to understand what's going on here. I
 don't actually think that the Cmm to LLVM pass is to blame. The faults
 seem to always occur in the std_blackhole function. I don't understand
 exactly how blackholes work, I think it's something like:

 I find a pointer to a closure, at the other end I might find:
     - a value, I'm done.
     - a closure, I evaluate that.
     - a blackhole, another HEC has beat me to this closure, so I'll wait
 for them to finish.

 Perhaps there's simply a barrier missing from std_blackhole.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15449#comment:20>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler