gh-114203: Optimise simple recursive critical sections (#128126)

Add a fast path to (single-mutex) critical section locking _iff_ the mutex
is already held by the currently active, top-most critical section of this
thread. This can matter a lot for indirectly recursive critical sections
without intervening critical sections.

3 files changed, 32 insertions, 7 deletions

diff --git a/Include/internal/pycore_critical_section.h b/Include/internal/pycore_critical_section.h
index 9ba2fce..e66d6d8 100644
--- a/Include/internal/pycore_critical_section.h
+++ b/Include/internal/pycore_critical_section.h
@@ -145,6 +145,12 @@ _PyCriticalSection_Pop(PyCriticalSection *c)
 static inline void
 _PyCriticalSection_End(PyCriticalSection *c)
 {
+    // If the mutex is NULL, we used the fast path in
+    // _PyCriticalSection_BeginSlow for locks already held in the top-most
+    // critical section, and we shouldn't unlock or pop this critical section.
+    if (c->_cs_mutex == NULL) {
+        return;
+    }
     PyMutex_Unlock(c->_cs_mutex);
     _PyCriticalSection_Pop(c);
 }
@@ -199,6 +205,14 @@ _PyCriticalSection2_Begin(PyCriticalSection2 *c, PyObject *a, PyObject *b)
 static inline void
 _PyCriticalSection2_End(PyCriticalSection2 *c)
 {
+    // if mutex1 is NULL, we used the fast path in
+    // _PyCriticalSection_BeginSlow for mutexes that are already held,
+    // which should only happen when mutex1 and mutex2 were the same mutex,
+    // and mutex2 should also be NULL.
+    if (c->_cs_base._cs_mutex == NULL) {
+        assert(c->_cs_mutex2 == NULL);
+        return;
+    }
     if (c->_cs_mutex2) {
         PyMutex_Unlock(c->_cs_mutex2);
     }
diff --git a/Misc/NEWS.d/next/Core_and_Builtins/2024-12-20-23-07-33.gh-issue-114203.84NgoW.rst b/Misc/NEWS.d/next/Core_and_Builtins/2024-12-20-23-07-33.gh-issue-114203.84NgoW.rst
new file mode 100644
index 0000000..6a9856e
--- /dev/null
+++ b/Misc/NEWS.d/next/Core_and_Builtins/2024-12-20-23-07-33.gh-issue-114203.84NgoW.rst
@@ -0,0 +1 @@
+Optimize ``Py_BEGIN_CRITICAL_SECTION`` for simple recursive calls.
diff --git a/Python/critical_section.c b/Python/critical_section.c
index 62ed255..73857b8 100644
--- a/Python/critical_section.c
+++ b/Python/critical_section.c
@@ -8,11 +8,28 @@ static_assert(_Alignof(PyCriticalSection) >= 4,
               "critical section must be aligned to at least 4 bytes");
 #endif
 
+#ifdef Py_GIL_DISABLED
+static PyCriticalSection *
+untag_critical_section(uintptr_t tag)
+{
+    return (PyCriticalSection *)(tag & ~_Py_CRITICAL_SECTION_MASK);
+}
+#endif
+
 void
 _PyCriticalSection_BeginSlow(PyCriticalSection *c, PyMutex *m)
 {
 #ifdef Py_GIL_DISABLED
     PyThreadState *tstate = _PyThreadState_GET();
+    // As an optimisation for locking the same object recursively, skip
+    // locking if the mutex is currently locked by the top-most critical
+    // section.
+    if (tstate->critical_section &&
+        untag_critical_section(tstate->critical_section)->_cs_mutex == m) {
+        c->_cs_mutex = NULL;
+        c->_cs_prev = 0;
+        return;
+    }
     c->_cs_mutex = NULL;
     c->_cs_prev = (uintptr_t)tstate->critical_section;
     tstate->critical_section = (uintptr_t)c;
@@ -42,13 +59,6 @@ _PyCriticalSection2_BeginSlow(PyCriticalSection2 *c, PyMutex *m1, PyMutex *m2,
 #endif
 }
 
-#ifdef Py_GIL_DISABLED
-static PyCriticalSection *
-untag_critical_section(uintptr_t tag)
-{
-    return (PyCriticalSection *)(tag & ~_Py_CRITICAL_SECTION_MASK);
-}
-#endif
 
 // Release all locks held by critical sections. This is called by
 // _PyThreadState_Detach.