gh-103323: Get the "Current" Thread State from a Thread-Local Variable (gh-103324)

We replace _PyRuntime.tstate_current with a thread-local variable. As part of this change, we add a _Py_thread_local macro in pyport.h (only for the core runtime) to smooth out the compiler differences. The main motivation here is in support of a per-interpreter GIL, but this change also provides some performance improvement opportunities.

Note that we do not provide a fallback to the thread-local, either falling back to the old tstate_current or to thread-specific storage (PyThread_tss_*()). If that proves problematic then we can circle back. I consider it unlikely, but will run the buildbots to double-check.

Also note that this does not change any of the code related to the GILState API, where it uses a thread state stored in thread-specific storage. I suspect we can combine that with _Py_tss_tstate (from here). However, that can be addressed separately and is not urgent (nor critical).

(While this change was mostly done independently, I did take some inspiration from earlier (~2020) work by @markshannon (main...markshannon:threadstate_in_tls) and @vstinner (#23976).)

1 files changed, 32 insertions, 6 deletions

diff --git a/Python/pystate.c b/Python/pystate.c
index 1e04887..d108cfc 100644
--- a/Python/pystate.c
+++ b/Python/pystate.c
@@ -60,23 +60,43 @@ extern "C" {
    For each of these functions, the GIL must be held by the current thread.
  */
 
+
+#ifdef HAVE_THREAD_LOCAL
+_Py_thread_local PyThreadState *_Py_tss_tstate = NULL;
+#endif
+
 static inline PyThreadState *
-current_fast_get(_PyRuntimeState *runtime)
+current_fast_get(_PyRuntimeState *Py_UNUSED(runtime))
 {
-    return (PyThreadState*)_Py_atomic_load_relaxed(&runtime->tstate_current);
+#ifdef HAVE_THREAD_LOCAL
+    return _Py_tss_tstate;
+#else
+    // XXX Fall back to the PyThread_tss_*() API.
+#  error "no supported thread-local variable storage classifier"
+#endif
 }
 
 static inline void
-current_fast_set(_PyRuntimeState *runtime, PyThreadState *tstate)
+current_fast_set(_PyRuntimeState *Py_UNUSED(runtime), PyThreadState *tstate)
 {
     assert(tstate != NULL);
-    _Py_atomic_store_relaxed(&runtime->tstate_current, (uintptr_t)tstate);
+#ifdef HAVE_THREAD_LOCAL
+    _Py_tss_tstate = tstate;
+#else
+    // XXX Fall back to the PyThread_tss_*() API.
+#  error "no supported thread-local variable storage classifier"
+#endif
 }
 
 static inline void
-current_fast_clear(_PyRuntimeState *runtime)
+current_fast_clear(_PyRuntimeState *Py_UNUSED(runtime))
 {
-    _Py_atomic_store_relaxed(&runtime->tstate_current, (uintptr_t)NULL);
+#ifdef HAVE_THREAD_LOCAL
+    _Py_tss_tstate = NULL;
+#else
+    // XXX Fall back to the PyThread_tss_*() API.
+#  error "no supported thread-local variable storage classifier"
+#endif
 }
 
 #define tstate_verify_not_active(tstate) \
@@ -84,6 +104,12 @@ current_fast_clear(_PyRuntimeState *runtime)
         _Py_FatalErrorFormat(__func__, "tstate %p is still current", tstate); \
     }
 
+PyThreadState *
+_PyThreadState_GetCurrent(void)
+{
+    return current_fast_get(&_PyRuntime);
+}
+
 
 //------------------------------------------------
 // the thread state bound to the current OS thread