Bug report

1. fi_freelist isn't thread-safe (move to pycore_freelist.h and follow that pattern)
2. enter_task, leave_task, and swap_current_task aren't thread-safe due to shared state->current_tasks and borrowed references.
3. register_task and unregister_task aren't thread-safe due to shared state->asyncio_tasks linked list
4. _asyncio_all_tasks_impl isn't thread-safe due the the asyncio_tasks linked list.

For 2, 3, and 4, we can consider using critical sections to protect the accesses to state->current_tasks and state->asyncio_tasks.

Longer term, moving data to per-loop will probably help with multi-threaded scaling.

Linked PRs

• gh-120974: Use common freelist code in asyncio #122132
• gh-120974: Make _asyncio._enter_task atomic in the free-threaded build #122138
• gh-120974: Make _asyncio._leave_task atomic in the free-threaded build #122139
• [3.13] gh-120974: Make _asyncio._enter_task atomic in the free-threaded build (GH-122138) #122152
• [3.13] gh-120974: Make _asyncio._leave_task atomic in the free-threaded build (GH-122139) #122186
• gh-120974: Make asyncio swap_current_task safe in free-threaded build #122317
• [3.13] gh-120974: Make asyncio swap_current_task safe in free-threaded build (GH-122317) #122612
• GH-120974: make _asyncio_all_tasks_impl thread safe #122801