Optimise wakeups for STM

Avoids repeated wakeup messages being sent when a TVar is written to
multiple times. See comments for details.

Test Plan:
* Test from #15136 (will be added to stm shortly)
* existing stm tests

Reviewers: bgamari, osa1, erikd

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

GHC Trac Issues: #15136

Differential Revision: https://phabricator.haskell.org/D4961

1 files changed, 23 insertions, 1 deletions

diff --git a/rts/STM.c b/rts/STM.c
index abb44172dd..976ad87616 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -332,7 +332,29 @@ 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.
 
-    tryWakeupThread(cap,tso);
+    // Only the capability that owns this TSO may unblock it. We can
+    // call tryWakeupThread() which will either unblock it directly if
+    // it belongs to this cap, or send a message to the owning cap
+    // otherwise.
+
+    // But we don't really want to send multiple messages if we write
+    // to the same TVar multiple times, and the owning cap hasn't yet
+    // woken up the thread and removed it from the TVar's watch list.
+    // So, we use the tso->block_info as a flag to indicate whether
+    // we've already done tryWakeupThread() for this thread.
+
+    // Safety Note: we hold the TVar lock at this point, so we know
+    // that this thread is definitely still blocked, since the first
+    // thing a thread will do when it runs is remove itself from the
+    // TVar watch queues, and to do that it would need to lock the
+    // TVar.
+
+    if (tso->block_info.closure != &stg_STM_AWOKEN_closure) {
+        // safe to do a non-atomic test-and-set here, because it's
+        // fine if we do multiple tryWakeupThread()s.
+        tso->block_info.closure = &stg_STM_AWOKEN_closure;
+        tryWakeupThread(cap,tso);
+    }
 }
 
 static void unpark_waiters_on(Capability *cap, StgTVar *s) {