Note.

src/c_api/coll_c_api.cc

@@ -174,6 +174,18 @@ XGB_DLL int XGTrackerFree(TrackerHandle handle) {
   common::Timer timer;
   timer.Start();
   // Make sure no one else is waiting on the tracker.
+
+  // Quote from https://en.cppreference.com/w/cpp/memory/shared_ptr/use_count#Notes:
+  //
+  // In multithreaded environment, `use_count() == 1` does not imply that the object is
+  // safe to modify because accesses to the managed object by former shared owners may not
+  // have completed, and because new shared owners may be introduced concurrently.
+  //
+  // - We don't have the first case since we never access the raw pointer.
+  //
+  // - We don't hve the second case for most of the scenarios since tracker is an unique
+  //   object, if the free function is called before another function calls, it's likely
+  //   to be a bug in the user code. The use_count should only decrease in this function.
   while (ptr->first.use_count() != 1) {
     auto ela = timer.Duration().count();
     if (collective::HasTimeout(ptr->first->Timeout()) && ela > ptr->first->Timeout().count()) {