Fix deadlock between STM and throwTo

There was a lock-order reversal between lockTSO() and the TVar lock,
see #15136 for the details.

It turns out we can fix this pretty easily by just deleting all the
locking code(!).  The principle for unblocking a `BlockedOnSTM` thread
then becomes the same as for other kinds of blocking: if the TSO
belongs to this capability then we do it directly, otherwise we send a
message to the capability that owns the TSO. That is, a thread blocked
on STM is owned by its capability, as it should be.

The possible downside of this is that we might send multiple messages
to wake up a thread when the thread is on another capability. This is
safe, it's just not very efficient.  I'll try to do some experiments
to see if this is a problem.

Test Plan: Test case from #15136 doesn't deadlock any more.

Reviewers: bgamari, osa1, erikd

Reviewed By: osa1

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15136

Differential Revision: https://phabricator.haskell.org/D4956

1 files changed, 4 insertions, 1 deletions

diff --git a/rts/Threads.c b/rts/Threads.c
index be6962246d..78c5b6cc84 100644
--- a/rts/Threads.c
+++ b/rts/Threads.c
@@ -297,8 +297,11 @@ tryWakeupThread (Capability *cap, StgTSO *tso)
         goto unblock;
     }
 
-    case BlockedOnBlackHole:
     case BlockedOnSTM:
+        tso->block_info.closure = &stg_STM_AWOKEN_closure;
+        goto unblock;
+
+    case BlockedOnBlackHole:
     case ThreadMigrating:
         goto unblock;