fix_concurrent_slot_xmin_update.patch

diff --git a/src/backend/replication/logical/logical.c b/src/backend/replication/logical/logical.c
index 1a58dd7649..607a605075 100644
--- a/src/backend/replication/logical/logical.c
+++ b/src/backend/replication/logical/logical.c
@@ -386,11 +386,11 @@ CreateInitDecodingContext(const char *plugin,
 	 * without further interlock its return value might immediately be out of
 	 * date.
 	 *
-	 * So we have to acquire the ProcArrayLock to prevent computation of new
-	 * xmin horizons by other backends, get the safe decoding xid, and inform
-	 * the slot machinery about the new limit. Once that's done the
-	 * ProcArrayLock can be released as the slot machinery now is
-	 * protecting against vacuum.
+	 * So we have to acquire both the ReplicationSlotControlLock and the
+	 * ProcArrayLock to prevent concurrent computation and update of new xmin
+	 * horizons by other backends, get the safe decoding xid, and inform the
+	 * slot machinery about the new limit. Once that's done the both locks
+	 * can be released as the slot machinery now is protecting against vacuum.
 	 *
 	 * Note that, temporarily, the data, not just the catalog, xmin has to be
 	 * reserved if a data snapshot is to be exported.  Otherwise the initial
@@ -403,6 +403,7 @@ CreateInitDecodingContext(const char *plugin,
 	 *
 	 * ----
 	 */
+	LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
 	LWLockAcquire(ProcArrayLock, LW_EXCLUSIVE);
 
 	xmin_horizon = GetOldestSafeDecodingTransactionId(!need_full_snapshot);
@@ -417,6 +418,7 @@ CreateInitDecodingContext(const char *plugin,
 	ReplicationSlotsComputeRequiredXmin(true);
 
 	LWLockRelease(ProcArrayLock);
+	LWLockRelease(ReplicationSlotControlLock);
 
 	ReplicationSlotMarkDirty();
 	ReplicationSlotSave();
diff --git a/src/backend/replication/slot.c b/src/backend/replication/slot.c
index f286918f69..852ec9564a 100644
--- a/src/backend/replication/slot.c
+++ b/src/backend/replication/slot.c
@@ -828,8 +828,11 @@ ReplicationSlotPersist(void)
 /*
  * Compute the oldest xmin across all slots and store it in the ProcArray.
  *
- * If already_locked is true, ProcArrayLock has already been acquired
- * exclusively.
+ * If already_locked is true, both the ReplicationSlotControlLock and
+ * the ProcArrayLock have already been acquired exclusively.
+ *
+ * Note that the ReplicationSlotControlLock must be locked first to avoid
+ * deadlocks.
  */
 void
 ReplicationSlotsComputeRequiredXmin(bool already_locked)
@@ -839,8 +842,26 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 	TransactionId agg_catalog_xmin = InvalidTransactionId;
 
 	Assert(ReplicationSlotCtl != NULL);
+	Assert(!already_locked ||
+		   (LWLockHeldByMeInMode(ReplicationSlotControlLock, LW_EXCLUSIVE) &&
+			LWLockHeldByMeInMode(ProcArrayLock, LW_EXCLUSIVE)));
 
-	LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
+	/*
+	 * Hold the ReplicationSlotControlLock exclusive until after updating
+	 * the slot xmin values, so no backend can compute and update the new
+	 * value concurrently.
+	 *
+	 * One might think that we can hold the ProcArrayLock exclusively,
+	 * compute the xmin values while holding the ReplicationSlotControlLock
+	 * in shared mode, and update the slot xmin values, but it could increase
+	 * lock contention on the ProcArrayLock, which is not great since this
+	 * function can be called at non-negligible frequency.
+	 *
+	 * We instead increase lock contention on the ReplicationSlotControlLock
+	 * but it would be less harmful.
+	 */
+	if (!already_locked)
+		LWLockAcquire(ReplicationSlotControlLock, LW_EXCLUSIVE);
 
 	for (i = 0; i < max_replication_slots; i++)
 	{
@@ -876,9 +897,10 @@ ReplicationSlotsComputeRequiredXmin(bool already_locked)
 			agg_catalog_xmin = effective_catalog_xmin;
 	}
 
-	LWLockRelease(ReplicationSlotControlLock);
-
 	ProcArraySetReplicationSlotXmin(agg_xmin, agg_catalog_xmin, already_locked);
+
+	if (!already_locked)
+		LWLockRelease(ReplicationSlotControlLock);
 }
 
 /*