gh-113743: Make the MRO cache thread-safe in free-threaded builds (#113930)

Makes _PyType_Lookup thread safe, including:
    Thread safety of the underlying cache.
    Make mutation of mro and type members thread safe
    Also _PyType_GetMRO and _PyType_GetBases are currently returning borrowed references which aren't safe.

Include/cpython/pyatomic.h

@@ -469,6 +469,9 @@ _Py_atomic_store_int_release(int *obj, int value);
 static inline int
 _Py_atomic_load_int_acquire(const int *obj);
 
+static inline uint32_t
+_Py_atomic_load_uint32_acquire(const uint32_t *obj);
+
 
 // --- _Py_atomic_fence ------------------------------------------------------
 

Include/cpython/pyatomic_gcc.h

@@ -495,6 +495,9 @@ static inline int
 _Py_atomic_load_int_acquire(const int *obj)
 { return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }
 
+static inline uint32_t
+_Py_atomic_load_uint32_acquire(const uint32_t *obj)
+{ return __atomic_load_n(obj, __ATOMIC_ACQUIRE); }
 
 // --- _Py_atomic_fence ------------------------------------------------------
 

Include/cpython/pyatomic_msc.h

@@ -938,6 +938,17 @@ _Py_atomic_load_int_acquire(const int *obj)
 #endif
 }
 
+static inline uint32_t
+_Py_atomic_load_uint32_acquire(const uint32_t *obj)
+{
+#if defined(_M_X64) || defined(_M_IX86)
+    return *(uint32_t volatile *)obj;
+#elif defined(_M_ARM64)
+    return (int)__ldar32((uint32_t volatile *)obj);
+#else
+#  error "no implementation of _Py_atomic_load_uint32_acquire"
+#endif
+}
 
 // --- _Py_atomic_fence ------------------------------------------------------
 

Include/cpython/pyatomic_std.h

@@ -870,6 +870,13 @@ _Py_atomic_load_int_acquire(const int *obj)
                                 memory_order_acquire);
 }
 
+static inline uint32_t
+_Py_atomic_load_uint32_acquire(const uint32_t *obj)
+{
+    _Py_USING_STD;
+    return atomic_load_explicit((const _Atomic(uint32_t)*)obj,
+                                memory_order_acquire);
+}
 
 
 // --- _Py_atomic_fence ------------------------------------------------------

Include/internal/pycore_critical_section.h

@@ -293,7 +293,8 @@ _PyCriticalSection_SuspendAll(PyThreadState *tstate);
 #ifdef Py_GIL_DISABLED
 
 static inline void
-_PyCriticalSection_AssertHeld(PyMutex *mutex) {
+_PyCriticalSection_AssertHeld(PyMutex *mutex)
+{
 #ifdef Py_DEBUG
     PyThreadState *tstate = _PyThreadState_GET();
     uintptr_t prev = tstate->critical_section;

Include/internal/pycore_lock.h

@@ -251,6 +251,39 @@ PyAPI_FUNC(void) _PyRWMutex_RUnlock(_PyRWMutex *rwmutex);
 PyAPI_FUNC(void) _PyRWMutex_Lock(_PyRWMutex *rwmutex);
 PyAPI_FUNC(void) _PyRWMutex_Unlock(_PyRWMutex *rwmutex);
 
+// Similar to linux seqlock: https://en.wikipedia.org/wiki/Seqlock
+// We use a sequence number to lock the writer, an even sequence means we're unlocked, an odd
+// sequence means we're locked.  Readers will read the sequence before attempting to read the
+// underlying data and then read the sequence number again after reading the data.  If the
+// sequence has not changed the data is valid.
+//
+// Differs a little bit in that we use CAS on sequence as the lock, instead of a seperate spin lock.
+// The writer can also detect that the undelering data has not changed and abandon the write
+// and restore the previous sequence.
+typedef struct {
+    uint32_t sequence;
+} _PySeqLock;
+
+// Lock the sequence lock for the writer
+PyAPI_FUNC(void) _PySeqLock_LockWrite(_PySeqLock *seqlock);
+
+// Unlock the sequence lock and move to the next sequence number.
+PyAPI_FUNC(void) _PySeqLock_UnlockWrite(_PySeqLock *seqlock);
+
+// Abandon the current update indicating that no mutations have occured
+// and restore the previous sequence value.
+PyAPI_FUNC(void) _PySeqLock_AbandonWrite(_PySeqLock *seqlock);
+
+// Begin a read operation and return the current sequence number.
+PyAPI_FUNC(uint32_t) _PySeqLock_BeginRead(_PySeqLock *seqlock);
+
+// End the read operation and confirm that the sequence number has not changed.
+// Returns 1 if the read was successful or 0 if the read should be re-tried.
+PyAPI_FUNC(uint32_t) _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous);
+
+// Check if the lock was held during a fork and clear the lock.  Returns 1
+// if the lock was held and any associated datat should be cleared.
+PyAPI_FUNC(uint32_t) _PySeqLock_AfterFork(_PySeqLock *seqlock);
 
 #ifdef __cplusplus
 }

Include/internal/pycore_typeobject.h

@@ -9,6 +9,7 @@ extern "C" {
 #endif
 
 #include "pycore_moduleobject.h"  // PyModuleObject
+#include "pycore_lock.h"          // PyMutex
 
 
 /* state */
@@ -21,6 +22,7 @@ struct _types_runtime_state {
     // bpo-42745: next_version_tag remains shared by all interpreters
     // because of static types.
     unsigned int next_version_tag;
+    PyMutex type_mutex;
 };
 
 
@@ -28,6 +30,9 @@ struct _types_runtime_state {
 // see _PyType_Lookup().
 struct type_cache_entry {
     unsigned int version;  // initialized from type->tp_version_tag
+#ifdef Py_GIL_DISABLED
+   _PySeqLock sequence;
+#endif
     PyObject *name;        // reference to exactly a str or None
     PyObject *value;       // borrowed reference or NULL
 };
@@ -74,7 +79,7 @@ struct types_state {
 extern PyStatus _PyTypes_InitTypes(PyInterpreterState *);
 extern void _PyTypes_FiniTypes(PyInterpreterState *);
 extern void _PyTypes_Fini(PyInterpreterState *);
-
+extern void _PyTypes_AfterFork(void);
 
 /* other API */