[GitHub] [arrow] westonpace commented on a diff in pull request #36094: GH-36092: [C++] Simplify concurrency in as-of-join node

["westonpace (via GitHub)" <gi...@apache.org>]

westonpace commented on code in PR #36094:
URL: https://github.com/apache/arrow/pull/36094#discussion_r1237684152


##########
cpp/src/arrow/acero/asof_join_node.cc:
##########
@@ -364,21 +364,16 @@ struct MemoStore {
     std::swap(index_, memo.index_);
 #endif
     std::swap(no_future_, memo.no_future_);
-    current_time_ = memo.current_time_.exchange(static_cast<OnType>(current_time_));
+    std::swap(current_time_, memo.current_time_);
     entries_.swap(memo.entries_);
     future_entries_.swap(memo.future_entries_);
     times_.swap(memo.times_);
   }
 
-  // Updates the current time to `ts` if it is less. A different thread may win the race
-  // to update the current time to more than `ts` but not to less. Returns whether the
-  // current time was changed from its value at the beginning of this invocation.
+  // Updates the current time to `ts` if it is less. Returns true if updated.
   bool UpdateTime(OnType ts) {
-    OnType prev_time = current_time_;
-    bool update = prev_time < ts;
-    while (prev_time < ts && !current_time_.compare_exchange_weak(prev_time, ts)) {
-      // intentionally empty - standard CAS loop
-    }
+    bool update = current_time_ < ts;
+    if (update) current_time_ = ts;

Review Comment:
   ```suggestion
       current_time_ = std::min(current_time_, ts);
   ```
   Minor nit: Maybe simpler?



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