gh-115103: Implement delayed free mechanism for free-threaded builds (#115367)

This adds `_PyMem_FreeDelayed()` and supporting functions. The
`_PyMem_FreeDelayed()` function frees memory with the same allocator as
`PyMem_Free()`, but after some delay to ensure that concurrent lock-free
readers have finished.

Objects/obmalloc.c

@@ -948,6 +948,196 @@ _PyMem_Strdup(const char *str)
     return copy;
 }
 
+/***********************************************/
+/* Delayed freeing support for Py_GIL_DISABLED */
+/***********************************************/
+
+// So that sizeof(struct _mem_work_chunk) is 4096 bytes on 64-bit platforms.
+#define WORK_ITEMS_PER_CHUNK 254
+
+// A pointer to be freed once the QSBR read sequence reaches qsbr_goal.
+struct _mem_work_item {
+    void *ptr;
+    uint64_t qsbr_goal;
+};
+
+// A fixed-size buffer of pointers to be freed
+struct _mem_work_chunk {
+    // Linked list node of chunks in queue
+    struct llist_node node;
+
+    Py_ssize_t rd_idx;  // index of next item to read
+    Py_ssize_t wr_idx;  // index of next item to write
+    struct _mem_work_item array[WORK_ITEMS_PER_CHUNK];
+};
+
+void
+_PyMem_FreeDelayed(void *ptr)
+{
+#ifndef Py_GIL_DISABLED
+    PyMem_Free(ptr);
+#else
+    if (_PyRuntime.stoptheworld.world_stopped) {
+        // Free immediately if the world is stopped, including during
+        // interpreter shutdown.
+        PyMem_Free(ptr);
+        return;
+    }
+
+    _PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
+    struct llist_node *head = &tstate->mem_free_queue;
+
+    struct _mem_work_chunk *buf = NULL;
+    if (!llist_empty(head)) {
+        // Try to re-use the last buffer
+        buf = llist_data(head->prev, struct _mem_work_chunk, node);
+        if (buf->wr_idx == WORK_ITEMS_PER_CHUNK) {
+            // already full
+            buf = NULL;
+        }
+    }
+
+    if (buf == NULL) {
+        buf = PyMem_Calloc(1, sizeof(*buf));
+        if (buf != NULL) {
+            llist_insert_tail(head, &buf->node);
+        }
+    }
+
+    if (buf == NULL) {
+        // failed to allocate a buffer, free immediately
+        _PyEval_StopTheWorld(tstate->base.interp);
+        PyMem_Free(ptr);
+        _PyEval_StartTheWorld(tstate->base.interp);
+        return;
+    }
+
+    assert(buf != NULL && buf->wr_idx < WORK_ITEMS_PER_CHUNK);
+    uint64_t seq = _Py_qsbr_deferred_advance(tstate->qsbr);
+    buf->array[buf->wr_idx].ptr = ptr;
+    buf->array[buf->wr_idx].qsbr_goal = seq;
+    buf->wr_idx++;
+
+    if (buf->wr_idx == WORK_ITEMS_PER_CHUNK) {
+        _PyMem_ProcessDelayed((PyThreadState *)tstate);
+    }
+#endif
+}
+
+static struct _mem_work_chunk *
+work_queue_first(struct llist_node *head)
+{
+    return llist_data(head->next, struct _mem_work_chunk, node);
+}
+
+static void
+process_queue(struct llist_node *head, struct _qsbr_thread_state *qsbr,
+              bool keep_empty)
+{
+    while (!llist_empty(head)) {
+        struct _mem_work_chunk *buf = work_queue_first(head);
+
+        while (buf->rd_idx < buf->wr_idx) {
+            struct _mem_work_item *item = &buf->array[buf->rd_idx];
+            if (!_Py_qsbr_poll(qsbr, item->qsbr_goal)) {
+                return;
+            }
+
+            PyMem_Free(item->ptr);
+            buf->rd_idx++;
+        }
+
+        assert(buf->rd_idx == buf->wr_idx);
+        if (keep_empty && buf->node.next == head) {
+            // Keep the last buffer in the queue to reduce re-allocations
+            buf->rd_idx = buf->wr_idx = 0;
+            return;
+        }
+
+        llist_remove(&buf->node);
+        PyMem_Free(buf);
+    }
+}
+
+static void
+process_interp_queue(struct _Py_mem_interp_free_queue *queue,
+                     struct _qsbr_thread_state *qsbr)
+{
+    if (!_Py_atomic_load_int_relaxed(&queue->has_work)) {
+        return;
+    }
+
+    // Try to acquire the lock, but don't block if it's already held.
+    if (_PyMutex_LockTimed(&queue->mutex, 0, 0) == PY_LOCK_ACQUIRED) {
+        process_queue(&queue->head, qsbr, false);
+
+        int more_work = !llist_empty(&queue->head);
+        _Py_atomic_store_int_relaxed(&queue->has_work, more_work);
+
+        PyMutex_Unlock(&queue->mutex);
+    }
+}
+
+void
+_PyMem_ProcessDelayed(PyThreadState *tstate)
+{
+    PyInterpreterState *interp = tstate->interp;
+    _PyThreadStateImpl *tstate_impl = (_PyThreadStateImpl *)tstate;
+
+    // Process thread-local work
+    process_queue(&tstate_impl->mem_free_queue, tstate_impl->qsbr, true);
+
+    // Process shared interpreter work
+    process_interp_queue(&interp->mem_free_queue, tstate_impl->qsbr);
+}
+
+void
+_PyMem_AbandonDelayed(PyThreadState *tstate)
+{
+    PyInterpreterState *interp = tstate->interp;
+    struct llist_node *queue = &((_PyThreadStateImpl *)tstate)->mem_free_queue;
+
+    if (llist_empty(queue)) {
+        return;
+    }
+
+    // Check if the queue contains one empty buffer
+    struct _mem_work_chunk *buf = work_queue_first(queue);
+    if (buf->rd_idx == buf->wr_idx) {
+        llist_remove(&buf->node);
+        PyMem_Free(buf);
+        assert(llist_empty(queue));
+        return;
+    }
+
+    // Merge the thread's work queue into the interpreter's work queue.
+    PyMutex_Lock(&interp->mem_free_queue.mutex);
+    llist_concat(&interp->mem_free_queue.head, queue);
+    _Py_atomic_store_int_relaxed(&interp->mem_free_queue.has_work, 1);
+    PyMutex_Unlock(&interp->mem_free_queue.mutex);
+
+    assert(llist_empty(queue));  // the thread's queue is now empty
+}
+
+void
+_PyMem_FiniDelayed(PyInterpreterState *interp)
+{
+    struct llist_node *head = &interp->mem_free_queue.head;
+    while (!llist_empty(head)) {
+        struct _mem_work_chunk *buf = work_queue_first(head);
+
+        while (buf->rd_idx < buf->wr_idx) {
+            // Free the remaining items immediately. There should be no other
+            // threads accessing the memory at this point during shutdown.
+            struct _mem_work_item *item = &buf->array[buf->rd_idx];
+            PyMem_Free(item->ptr);
+            buf->rd_idx++;
+        }
+
+        llist_remove(&buf->node);
+        PyMem_Free(buf);
+    }
+}
 
 /**************************/
 /* the "object" allocator */