Re: [PATCH] io_uring: don't issue reqs in iopoll mode when ctx is dying

[Xiaoguang Wang <xiaoguang.wang@linux.alibaba.com>]

hi,

> On 2/23/21 7:30 PM, Xiaoguang Wang wrote:
>> hi Pavel,
>>
>>> On 08/02/2021 13:35, Pavel Begunkov wrote:
>>>> On 08/02/2021 02:50, Xiaoguang Wang wrote:
>>>>>>> The io_identity's count is underflowed. It's because in io_put_identity,
>>>>>>> first argument tctx comes from req->task->io_uring, the second argument
>>>>>>> comes from the task context that calls io_req_init_async, so the compare
>>>>>>> in io_put_identity maybe meaningless. See below case:
>>>>>>>        task context A issue one polled req, then req->task = A.
>>>>>>>        task context B do iopoll, above req returns with EAGAIN error.
>>>>>>>        task context B re-issue req, call io_queue_async_work for req.
>>>>>>>        req->task->io_uring will set to task context B's identity, or cow new one.
>>>>>>> then for above case, in io_put_identity(), the compare is meaningless.
>>>>>>>
>>>>>>> IIUC, req->task should indicates the initial task context that issues req,
>>>>>>> then if it gets EAGAIN error, we'll call io_prep_async_work() in req->task
>>>>>>> context, but iopoll reqs seems special, they maybe issued successfully and
>>>>>>> got re-issued in other task context because of EAGAIN error.
>>>>>>
>>>>>> Looks as you say, but the patch doesn't solve the issue completely.
>>>>>> 1. We must not do io_queue_async_work() under a different task context,
>>>>>> because of it potentially uses a different set of resources. So, I just
>>>>>> thought that it would be better to punt it to the right task context
>>>>>> via task_work. But...
>>>>>>
>>>>>> 2. ...iovec import from io_resubmit_prep() might happen after submit ends,
>>>>>> i.e. when iovec was freed in userspace. And that's not great at all.
>>>>> Yes, agree, that's why I say we neeed to re-consider the io identity codes
>>>>> more in commit message :) I'll have a try to prepare a better one.
>>>>
>>>> I'd vote for dragging -AGAIN'ed reqs that don't need io_import_iovec()
>>>> through task_work for resubmission, and fail everything else. Not great,
>>>> but imho better than always setting async_data.
>>>
>>> Hey Xiaoguang, are you working on this? I would like to leave it to you,
>>> If you do.
>> Sorry, currently I'm busy with other project and don't have much time to work on
>> it yet. Hao Xu will help to continue work on the new version patch.
> 
> Is it issue or reissue? I found this one today:
> 
> https://lore.kernel.org/io-uring/c9f6e1f6-ff82-0e58-ab66-956d0cde30ff@kernel.dk/
Yeah, my initial patch is similar to yours, but it only solves the bug described
in my commit message partially(ctx is dying), you can have a look at my commit message
for the bug bug scene, thanks.

Regards,
Xiaoguang Wang


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