Re: [PATCH 2/2] io_uring: don't hold uring_lock when calling io_run_task_work*

Pavel Begunkov
2021-02-05T10:25:56Z

On 05/02/2021 09:57, Hao Xu wrote:
> 在 2021/2/4 下午11:26, Pavel Begunkov 写道:
>> On 04/02/2021 11:17, Pavel Begunkov wrote:
>>> On 04/02/2021 03:25, Hao Xu wrote:
>>>> 在 2021/2/4 上午12:45, Pavel Begunkov 写道:
>>>>> On 03/02/2021 16:35, Pavel Begunkov wrote:
>>>>>> On 03/02/2021 14:57, Hao Xu wrote:
>>>>>>> This is caused by calling io_run_task_work_sig() to do work under
>>>>>>> uring_lock while the caller io_sqe_files_unregister() already held
>>>>>>> uring_lock.
>>>>>>> we need to check if uring_lock is held by us when doing unlock around
>>>>>>> io_run_task_work_sig() since there are code paths down to that place
>>>>>>> without uring_lock held.
>>>>>>
>>>>>> 1. we don't want to allow parallel io_sqe_files_unregister()s
>>>>>> happening, it's synchronised by uring_lock atm. Otherwise it's
>>>>>> buggy.
>>>> Here "since there are code paths down to that place without uring_lock held" I mean code path of io_ring_ctx_free().
>>>
>>> I guess it's to the 1/2, but let me outline the problem again:
>>> if you have two tasks userspace threads sharing a ring, then they
>>> can both and in parallel call syscall:files_unregeister. That's
>>> a potential double percpu_ref_kill(&data->refs), or even worse.
>>>
>>> Same for 2, but racing for the table and refs.
>>
>> There is a couple of thoughts for this:
>>
>> 1. I don't like waiting without holding the lock in general, because
>> someone can submit more reqs in-between and so indefinitely postponing
>> the files_unregister.
> Thanks, Pavel.
> I thought this issue before, until I saw this in __io_uring_register:
>
>   if (io_register_op_must_quiesce(opcode)) {
>           percpu_ref_kill(&ctx->refs);

It is different because of this kill, it will prevent submissions.

>
>           /*
>           ¦* Drop uring mutex before waiting for references to exit. If
>           ¦* another thread is currently inside io_uring_enter() it might
>           ¦* need to grab the uring_lock to make progress. If we hold it
>           ¦* here across the drain wait, then we can deadlock. It's safe
>           ¦* to drop the mutex here, since no new references will come in
>           ¦* after we've killed the percpu ref.
>           ¦*/
>           mutex_unlock(&ctx->uring_lock);
>           do {
>                   ret = wait_for_completion_interruptible(&ctx->ref_comp);
>                   if (!ret)
>                           break;
>                   ret = io_run_task_work_sig();
>                   if (ret < 0)
>                           break;
>           } while (1);
>
>           mutex_lock(&ctx->uring_lock);
>
>           if (ret) {
>                   percpu_ref_resurrect(&ctx->refs);
>                   goto out_quiesce;
>           }
>   }
>
> So now I guess the postponement issue also exits in the above code since
> there could be another thread submiting reqs to the shared ctx(or we can say uring fd).
>
>> 2. I wouldn't want to add checks for that in submission path.
>>
>> So, a solution I think about is to wait under the lock, If we need to
>> run task_works -- briefly drop the lock, run task_works and then do
>> all unregister all over again. Keep an eye on refs, e.g. probably
>> need to resurrect it.
>>
>> Because we current task is busy nobody submits new requests on
>> its behalf, and so there can't be infinite number of in-task_work
>> reqs, and eventually it will just go wait/sleep forever (if not
>> signalled) under the mutex, so we can a kind of upper bound on
>> time.
>>
> Do you mean sleeping with timeout rather than just sleeping? I think this works, I'll work on this and think about the detail.

Without timeout -- it will be awaken when new task_works are coming in,
but Jens knows better.

> But before addressing this issue, Should I first send a patch to just fix the deadlock issue?

Do you mean the deadlock 2/2 was trying to fix? Or some else? The thread
is all about fixing it, but doing it right. Not sure there is a need for
faster but incomplete solution, if that's what you meant.

--
Pavel Begunkov