[3.13] gh-119344: Make critical section API public (GH-119353) (#120856)

This makes the following macros public as part of the non-limited C-API for
locking a single object or two objects at once.

* `Py_BEGIN_CRITICAL_SECTION(op)` / `Py_END_CRITICAL_SECTION()`
* `Py_BEGIN_CRITICAL_SECTION2(a, b)` / `Py_END_CRITICAL_SECTION2()`

The supporting functions and structs used by the macros are also exposed for
cases where C macros are not available.
(cherry picked from commit 8f17d69)

Doc/c-api/init.rst

@@ -2202,3 +2202,107 @@ The C-API provides a basic mutual exclusion lock.
    issue a fatal error.
 
    .. versionadded:: 3.13
+
+.. _python-critical-section-api:
+
+Python Critical Section API
+---------------------------
+
+The critical section API provides a deadlock avoidance layer on top of
+per-object locks for :term:`free-threaded <free threading>` CPython.  They are
+intended to replace reliance on the :term:`global interpreter lock`, and are
+no-ops in versions of Python with the global interpreter lock.
+
+Critical sections avoid deadlocks by implicitly suspending active critical
+sections and releasing the locks during calls to :c:func:`PyEval_SaveThread`.
+When :c:func:`PyEval_RestoreThread` is called, the most recent critical section
+is resumed, and its locks reacquired.  This means the critical section API
+provides weaker guarantees than traditional locks -- they are useful because
+their behavior is similar to the :term:`GIL`.
+
+The functions and structs used by the macros are exposed for cases
+where C macros are not available. They should only be used as in the
+given macro expansions. Note that the sizes and contents of the structures may
+change in future Python versions.
+
+.. note::
+
+   Operations that need to lock two objects at once must use
+   :c:macro:`Py_BEGIN_CRITICAL_SECTION2`.  You *cannot* use nested critical
+   sections to lock more than one object at once, because the inner critical
+   section may suspend the outer critical sections.  This API does not provide
+   a way to lock more than two objects at once.
+
+Example usage::
+
+   static PyObject *
+   set_field(MyObject *self, PyObject *value)
+   {
+      Py_BEGIN_CRITICAL_SECTION(self);
+      Py_SETREF(self->field, Py_XNewRef(value));
+      Py_END_CRITICAL_SECTION();
+      Py_RETURN_NONE;
+   }
+
+In the above example, :c:macro:`Py_SETREF` calls :c:macro:`Py_DECREF`, which
+can call arbitrary code through an object's deallocation function.  The critical
+section API avoids potentital deadlocks due to reentrancy and lock ordering
+by allowing the runtime to temporarily suspend the critical section if the
+code triggered by the finalizer blocks and calls :c:func:`PyEval_SaveThread`.
+
+.. c:macro:: Py_BEGIN_CRITICAL_SECTION(op)
+
+   Acquires the per-object lock for the object *op* and begins a
+   critical section.
+
+   In the free-threaded build, this macro expands to::
+
+      {
+          PyCriticalSection _py_cs;
+          PyCriticalSection_Begin(&_py_cs, (PyObject*)(op))
+
+   In the default build, this macro expands to ``{``.
+
+   .. versionadded:: 3.13
+
+.. c:macro:: Py_END_CRITICAL_SECTION()
+
+   Ends the critical section and releases the per-object lock.
+
+   In the free-threaded build, this macro expands to::
+
+          PyCriticalSection_End(&_py_cs);
+      }
+
+   In the default build, this macro expands to ``}``.
+
+   .. versionadded:: 3.13
+
+.. c:macro:: Py_BEGIN_CRITICAL_SECTION2(a, b)
+
+   Acquires the per-objects locks for the objects *a* and *b* and begins a
+   critical section.  The locks are acquired in a consistent order (lowest
+   address first) to avoid lock ordering deadlocks.
+
+   In the free-threaded build, this macro expands to::
+
+      {
+          PyCriticalSection2 _py_cs2;
+          PyCriticalSection_Begin2(&_py_cs2, (PyObject*)(a), (PyObject*)(b))
+
+   In the default build, this macro expands to ``{``.
+
+   .. versionadded:: 3.13
+
+.. c:macro:: Py_END_CRITICAL_SECTION2()
+
+   Ends the critical section and releases the per-object locks.
+
+   In the free-threaded build, this macro expands to::
+
+          PyCriticalSection_End2(&_py_cs2);
+      }
+
+   In the default build, this macro expands to ``}``.
+
+   .. versionadded:: 3.13

Include/Python.h

@@ -124,6 +124,7 @@
 #include "import.h"
 #include "abstract.h"
 #include "bltinmodule.h"
+#include "critical_section.h"
 #include "cpython/pyctype.h"
 #include "pystrtod.h"
 #include "pystrcmp.h"

Include/cpython/critical_section.h

@@ -0,0 +1,134 @@
+#ifndef Py_CPYTHON_CRITICAL_SECTION_H
+#  error "this header file must not be included directly"
+#endif
+
+// Python critical sections
+//
+// Conceptually, critical sections are a deadlock avoidance layer on top of
+// per-object locks. These helpers, in combination with those locks, replace
+// our usage of the global interpreter lock to provide thread-safety for
+// otherwise thread-unsafe objects, such as dict.
+//
+// NOTE: These APIs are no-ops in non-free-threaded builds.
+//
+// Straightforward per-object locking could introduce deadlocks that were not
+// present when running with the GIL. Threads may hold locks for multiple
+// objects simultaneously because Python operations can nest. If threads were
+// to acquire the same locks in different orders, they would deadlock.
+//
+// One way to avoid deadlocks is to allow threads to hold only the lock (or
+// locks) for a single operation at a time (typically a single lock, but some
+// operations involve two locks). When a thread begins a nested operation it
+// could suspend the locks for any outer operation: before beginning the nested
+// operation, the locks for the outer operation are released and when the
+// nested operation completes, the locks for the outer operation are
+// reacquired.
+//
+// To improve performance, this API uses a variation of the above scheme.
+// Instead of immediately suspending locks any time a nested operation begins,
+// locks are only suspended if the thread would block. This reduces the number
+// of lock acquisitions and releases for nested operations, while still
+// avoiding deadlocks.
+//
+// Additionally, the locks for any active operation are suspended around
+// other potentially blocking operations, such as I/O. This is because the
+// interaction between locks and blocking operations can lead to deadlocks in
+// the same way as the interaction between multiple locks.
+//
+// Each thread's critical sections and their corresponding locks are tracked in
+// a stack in `PyThreadState.critical_section`. When a thread calls
+// `_PyThreadState_Detach()`, such as before a blocking I/O operation or when
+// waiting to acquire a lock, the thread suspends all of its active critical
+// sections, temporarily releasing the associated locks. When the thread calls
+// `_PyThreadState_Attach()`, it resumes the top-most (i.e., most recent)
+// critical section by reacquiring the associated lock or locks.  See
+// `_PyCriticalSection_Resume()`.
+//
+// NOTE: Only the top-most critical section is guaranteed to be active.
+// Operations that need to lock two objects at once must use
+// `Py_BEGIN_CRITICAL_SECTION2()`. You *CANNOT* use nested critical sections
+// to lock more than one object at once, because the inner critical section
+// may  suspend the outer critical sections. This API does not provide a way
+// to lock more than two objects at once (though it could be added later
+// if actually needed).
+//
+// NOTE: Critical sections implicitly behave like reentrant locks because
+// attempting to acquire the same lock will suspend any outer (earlier)
+// critical sections. However, they are less efficient for this use case than
+// purposefully designed reentrant locks.
+//
+// Example usage:
+//  Py_BEGIN_CRITICAL_SECTION(op);
+//  ...
+//  Py_END_CRITICAL_SECTION();
+//
+// To lock two objects at once:
+//  Py_BEGIN_CRITICAL_SECTION2(op1, op2);
+//  ...
+//  Py_END_CRITICAL_SECTION2();
+
+typedef struct PyCriticalSection PyCriticalSection;
+typedef struct PyCriticalSection2 PyCriticalSection2;
+
+PyAPI_FUNC(void)
+PyCriticalSection_Begin(PyCriticalSection *c, PyObject *op);
+
+PyAPI_FUNC(void)
+PyCriticalSection_End(PyCriticalSection *c);
+
+PyAPI_FUNC(void)
+PyCriticalSection2_Begin(PyCriticalSection2 *c, PyObject *a, PyObject *b);
+
+PyAPI_FUNC(void)
+PyCriticalSection2_End(PyCriticalSection2 *c);
+
+#ifndef Py_GIL_DISABLED
+# define Py_BEGIN_CRITICAL_SECTION(op)      \
+    {
+# define Py_END_CRITICAL_SECTION()          \
+    }
+# define Py_BEGIN_CRITICAL_SECTION2(a, b)   \
+    {
+# define Py_END_CRITICAL_SECTION2()         \
+    }
+#else /* !Py_GIL_DISABLED */
+
+// NOTE: the contents of this struct are private and may change betweeen
+// Python releases without a deprecation period.
+struct PyCriticalSection {
+    // Tagged pointer to an outer active critical section (or 0).
+    uintptr_t _cs_prev;
+
+    // Mutex used to protect critical section
+    PyMutex *_cs_mutex;
+};
+
+// A critical section protected by two mutexes. Use
+// Py_BEGIN_CRITICAL_SECTION2 and Py_END_CRITICAL_SECTION2.
+// NOTE: the contents of this struct are private and may change betweeen
+// Python releases without a deprecation period.
+struct PyCriticalSection2 {
+    PyCriticalSection _cs_base;
+
+    PyMutex *_cs_mutex2;
+};
+
+# define Py_BEGIN_CRITICAL_SECTION(op)                                  \
+    {                                                                   \
+        PyCriticalSection _py_cs;                                       \
+        PyCriticalSection_Begin(&_py_cs, _PyObject_CAST(op))
+
+# define Py_END_CRITICAL_SECTION()                                      \
+        PyCriticalSection_End(&_py_cs);                                 \
+    }
+
+# define Py_BEGIN_CRITICAL_SECTION2(a, b)                               \
+    {                                                                   \
+        PyCriticalSection2 _py_cs2;                                     \
+        PyCriticalSection2_Begin(&_py_cs2, _PyObject_CAST(a), _PyObject_CAST(b))
+
+# define Py_END_CRITICAL_SECTION2()                                     \
+        PyCriticalSection2_End(&_py_cs2);                               \
+    }
+
+#endif

Include/critical_section.h

@@ -0,0 +1,16 @@
+#ifndef Py_CRITICAL_SECTION_H
+#define Py_CRITICAL_SECTION_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+#ifndef Py_LIMITED_API
+#  define Py_CPYTHON_CRITICAL_SECTION_H
+#  include "cpython/critical_section.h"
+#  undef Py_CPYTHON_CRITICAL_SECTION_H
+#endif
+
+#ifdef __cplusplus
+}
+#endif
+#endif /* !Py_CRITICAL_SECTION_H */