Class `asyncio.Semaphore` has dequeue in implementation for waiters of semaphore and seems like the intention of this dequeue is to maintain FIFO queue of waiters. But it doesn't work, coroutine that releases semaphore can acquire semaphore again. Below is reproducible example: ```python import asyncio async def process(idx: int, semaphore: asyncio.Semaphore) -> None: while True: async with semaphore: print(f'ACQUIRE {idx}') await asyncio.sleep(1) async def main() -> None: semaphore = asyncio.Semaphore(5) await asyncio.gather(*[process(idx, semaphore) for idx in range(20)]) asyncio.run(main()) ``` In console: ``` /home/y.hyzyla/Work/asynciosemaphorebug/venv/bin/python /home/y.hyzyla/Work/asynciosemaphorebug/main.py ACQUIRE 0 ACQUIRE 1 ACQUIRE 2 ACQUIRE 3 ACQUIRE 4 ACQUIRE 0 ACQUIRE 1 ACQUIRE 2 ACQUIRE 3 ACQUIRE 4 ACQUIRE 0 ACQUIRE 1 ACQUIRE 2 ACQUIRE 3 ACQUIRE 4 .... ``` Ugly fix, is to add asyncio.sleep right before semaphore. ```python while True: await asyncio.sleep(0) async with semaphore: ... ``` Also, I found a comment on Stack Overflow about race condition in Semaphore implementation, but I don't know about the quality of that comment: https://stackoverflow.com/questions/55951233/does-pythons-asyncio-lock-acquire-maintain-order#comment112978366_55951304

Good point. Currently, asyncio lock objects don't provide a strong FIFO guarantee. In a tight loop, a task can re-acquire the lock just after releasing even if there are pending waiters that were scheduled earlier. It's true also for Lock, Conditional, Event, etc. The solution requires *async* release method. Since the change is not backward compatible, a new method should be added, e.g. `await sem.release_and_wait()` for endorsing the context switch if there are pending waiters. async context manager can be modified for using the new method without backward compatibility problems easily. A hero who can help is welcome!

Or, maybe, there is a way to do everything without changing public API. The idea is: _wake_up_next can create a future which is set by *waked up task* on its acquiring. acquire method should wait for this future first before entering in `while self._value < 0:` loop. If the future is cancelled, `_wake_up_next` should be called again and waiting for the next future acquiring to be performed. If there is no *acquire waiting* future exists -- do everything as usual. All other lock objects should be modified also.

Thanks for response! > A hero who can help is welcome! I will try to make PR :raising_hand

Andrew, the same problem exists in asyncio.Queue which is also critical. Here's how I fixed it in edgedb code base: https://github.com/edgedb/edgedb/blob/08e41341024828df22a01cd690b11fcff00bca5e/edb/server/compiler_pool/queue.py#L51-L74

Thanks, Yuri. I'll take a look.

 New changeset 32e77154ddfc514a3144d5912bffdd957246fd6c by Andrew Svetlov in branch 'main': bpo-45997: Fix asyncio.Semaphore re-acquiring order (GH-31910) https://github.com/python/cpython/commit/32e77154ddfc514a3144d5912bffdd957246fd6c 

 New changeset 9d59381a5d20157930bae34e5f5a7bc5ef09fa89 by Miss Islington (bot) in branch '3.10': [3.10] bpo-45997: Fix asyncio.Semaphore re-acquiring order (GH-31910) (#32047) https://github.com/python/cpython/commit/9d59381a5d20157930bae34e5f5a7bc5ef09fa89 

 New changeset f47984b560f1dafe4d907cef4edadfb1746bf027 by Andrew Svetlov in branch '3.9': [3.9] bpo-45997: Fix asyncio.Semaphore re-acquiring order (GH-31910) (GH-32049) https://github.com/python/cpython/commit/f47984b560f1dafe4d907cef4edadfb1746bf027