Use int32_t for sequence lock

Use atomic storage instead of exchange
Use relaxed load when starting sequence lock for write
Fix some formatting

Author: DinoV

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_lock.h

@@ -261,7 +261,7 @@ PyAPI_FUNC(void) _PyRWMutex_Unlock(_PyRWMutex *rwmutex);
 // The writer can also detect that the undelering data has not changed and abandon the write
 // and restore the previous sequence.
 typedef struct {
-    int sequence;
+    uint32_t sequence;
 } _PySeqLock;
 
 // Lock the sequence lock for the writer
@@ -275,15 +275,15 @@ PyAPI_FUNC(void) _PySeqLock_UnlockWrite(_PySeqLock *seqlock);
 PyAPI_FUNC(void) _PySeqLock_AbandonWrite(_PySeqLock *seqlock);
 
 // Begin a read operation and return the current sequence number.
-PyAPI_FUNC(int) _PySeqLock_BeginRead(_PySeqLock *seqlock);
+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(int) _PySeqLock_EndRead(_PySeqLock *seqlock, int previous);
+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(int) _PySeqLock_AfterFork(_PySeqLock *seqlock);
+PyAPI_FUNC(uint32_t) _PySeqLock_AfterFork(_PySeqLock *seqlock);
 
 #ifdef __cplusplus
 }

Objects/typeobject.c

@@ -59,13 +59,13 @@ class object "PyObject *" "&PyBaseObject_Type"
 // in odd behaviors w.r.t. running with the GIL as the outer type lock could
 // be released and reacquired during a subclass update if there's contention
 // on the subclass lock.
-#define BEGIN_TYPE_LOCK()                                           \
-    {                                                               \
-    _PyCriticalSection _cs;                                         \
-    _PyCriticalSection_Begin(&_cs, &_PyRuntime.types.type_mutex);   \
+#define BEGIN_TYPE_LOCK()                                               \
+    {                                                                   \
+        _PyCriticalSection _cs;                                         \
+        _PyCriticalSection_Begin(&_cs, &_PyRuntime.types.type_mutex);   \
 
-#define END_TYPE_LOCK()                                             \
-        _PyCriticalSection_End(&_cs);                               \
+#define END_TYPE_LOCK()                                                 \
+        _PyCriticalSection_End(&_cs);                                   \
     }
 
 #define ASSERT_TYPE_LOCK_HELD() \

Python/lock.c

@@ -465,59 +465,61 @@ _PyRWMutex_Unlock(_PyRWMutex *rwmutex)
 void _PySeqLock_LockWrite(_PySeqLock *seqlock)
 {
     // lock the entry by setting by moving to an odd sequence number
-    int prev = seqlock->sequence;
+    uint32_t prev = _Py_atomic_load_uint32_relaxed(&seqlock->sequence);
     while (1) {
         if (SEQLOCK_IS_UPDATING(prev)) {
             // Someone else is currently updating the cache
             _Py_yield();
-            prev = _Py_atomic_load_int32_relaxed(&seqlock->sequence);
-        } else if (_Py_atomic_compare_exchange_int32(&seqlock->sequence, &prev, prev + 1)) {
+            prev = _Py_atomic_load_uint32_relaxed(&seqlock->sequence);
+        }
+        else if (_Py_atomic_compare_exchange_uint32(&seqlock->sequence, &prev, prev + 1)) {
             // We've locked the cache
             break;
-        } else {
+        }
+        else {
             _Py_yield();
         }
     }
 }
 
 void _PySeqLock_AbandonWrite(_PySeqLock *seqlock)
 {
-    int new_seq = seqlock->sequence - 1;
+    uint32_t new_seq = seqlock->sequence - 1;
     assert(!SEQLOCK_IS_UPDATING(new_seq));
-    _Py_atomic_exchange_int32(&seqlock->sequence, new_seq);
+    _Py_atomic_store_uint32(&seqlock->sequence, new_seq);
 }
 
 void _PySeqLock_UnlockWrite(_PySeqLock *seqlock)
 {
-    int new_seq = seqlock->sequence + 1;
+    uint32_t new_seq = seqlock->sequence + 1;
     assert(!SEQLOCK_IS_UPDATING(new_seq));
-    _Py_atomic_exchange_int32(&seqlock->sequence, new_seq);
+    _Py_atomic_store_uint32(&seqlock->sequence, new_seq);
 }
 
-int _PySeqLock_BeginRead(_PySeqLock *seqlock)
+uint32_t _PySeqLock_BeginRead(_PySeqLock *seqlock)
 {
-    int sequence = _Py_atomic_load_int_acquire(&seqlock->sequence);
+    uint32_t sequence = _Py_atomic_load_uint32_acquire(&seqlock->sequence);
     while (SEQLOCK_IS_UPDATING(sequence)) {
         _Py_yield();
-        sequence = _Py_atomic_load_int_acquire(&seqlock->sequence);
+        sequence = _Py_atomic_load_uint32_acquire(&seqlock->sequence);
     }
 
     return sequence;
 }
 
-int _PySeqLock_EndRead(_PySeqLock *seqlock, int previous)
+uint32_t _PySeqLock_EndRead(_PySeqLock *seqlock, uint32_t previous)
 {
     // Synchronize again and validate that the entry hasn't been updated
     // while we were readying the values.
-     if (_Py_atomic_load_int_acquire(&seqlock->sequence) == previous) {
+     if (_Py_atomic_load_uint32_acquire(&seqlock->sequence) == previous) {
         return 1;
      }
 
      _Py_yield();
      return 0;
 }
 
-int _PySeqLock_AfterFork(_PySeqLock *seqlock)
+uint32_t _PySeqLock_AfterFork(_PySeqLock *seqlock)
 {
     // Synchronize again and validate that the entry hasn't been updated
     // while we were readying the values.