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, 1 insertions, 18 deletions

diff --git a/rts/STM.c b/rts/STM.c
index 058eec7409..abb44172dd 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -332,24 +332,7 @@ static void unpark_tso(Capability *cap, StgTSO *tso) {
     // queues: it's up to the thread itself to remove it from the wait queues
     // if it decides to do so when it is scheduled.
 
-    // Unblocking a TSO from BlockedOnSTM is done under the TSO lock,
-    // to avoid multiple CPUs unblocking the same TSO, and also to
-    // synchronise with throwTo(). The first time the TSO is unblocked
-    // we mark this fact by setting block_info.closure == STM_AWOKEN.
-    // This way we can avoid sending further wakeup messages in the
-    // future.
-    lockTSO(tso);
-    if (tso->why_blocked == BlockedOnSTM &&
-        tso->block_info.closure == &stg_STM_AWOKEN_closure) {
-      TRACE("unpark_tso already woken up tso=%p", tso);
-    } else if (tso -> why_blocked == BlockedOnSTM) {
-      TRACE("unpark_tso on tso=%p", tso);
-      tso->block_info.closure = &stg_STM_AWOKEN_closure;
-      tryWakeupThread(cap,tso);
-    } else {
-      TRACE("spurious unpark_tso on tso=%p", tso);
-    }
-    unlockTSO(tso);
+    tryWakeupThread(cap,tso);
 }
 
 static void unpark_waiters_on(Capability *cap, StgTVar *s) {