Add a warning comment

Author: XuehaiPan

include/pybind11/detail/internals.h

@@ -602,6 +602,15 @@ class internals_pp_manager {
     /// acquire the GIL. Will never return nullptr.
     std::unique_ptr<InternalsType> *get_pp() {
 #ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
+        // WARNING: We cannot use `get_num_interpreters_seen() > 1` here to create a fast path for
+        //          the single-interpreter case.
+        // For multi-interpreter support, the subinterpreters can be initialized concurrently, such
+        // as using `InterpreterPoolExecutor` in Python. The first time this function may not be
+        // called in the main interpreter, e.g., a clean main interpreter that does not import any
+        // pybind11 module. Multiple subinterpreters may observe `get_num_interpreters_seen() <= 1`
+        // at the same time, while `get_num_interpreters_seen() += 1` in `PYBIND11_MODULE(...)` is
+        // called later.
+
         // Whenever the interpreter changes on the current thread we need to invalidate the
         // internals_pp so that it can be pulled from the interpreter's state dict.  That is
         // slow, so we use the current PyThreadState to check if it is necessary.