From a64e94f98ca18e53ecc13f736d50b9cb2d156b05 Mon Sep 17 00:00:00 2001
From: Ben Gamari <ben@smart-cactus.org>
Date: Fri, 21 Aug 2020 11:55:43 -0400
Subject: rts/nonmoving: Add missing STM write barrier

When updating a TRec for a TVar already part of a transaction we
previously neglected to add the old value to the update remembered set.
I suspect this was the cause of #18587.
---
 rts/STM.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/rts/STM.c b/rts/STM.c
index 5f72c0ebfb..cff0d55082 100644
--- a/rts/STM.c
+++ b/rts/STM.c
@@ -1342,6 +1342,9 @@ void stmWriteTVar(Capability *cap,
   if (entry != NULL) {
     if (entry_in == trec) {
       // Entry found in our trec
+      IF_NONMOVING_WRITE_BARRIER_ENABLED {
+        updateRemembSetPushClosure(cap, (StgClosure *) entry->new_value);
+      }
       entry -> new_value = new_value;
     } else {
       // Entry found in another trec
-- 
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