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From: Pavel Begunkov <[email protected]>
To: Xiaoguang Wang <[email protected]>,
	[email protected]
Cc: [email protected], [email protected]
Subject: Re: [PATCH] io_uring: don't issue reqs in iopoll mode when ctx is dying
Date: Mon, 8 Feb 2021 13:35:07 +0000	[thread overview]
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>

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.

-- 
Pavel Begunkov

  reply	other threads:[~2021-02-08 13:39 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-06 15:00 [PATCH] io_uring: don't issue reqs in iopoll mode when ctx is dying Xiaoguang Wang
2021-02-07 17:30 ` Pavel Begunkov
2021-02-08  2:50   ` Xiaoguang Wang
2021-02-08 13:35     ` Pavel Begunkov [this message]
2021-02-22 13:23       ` Pavel Begunkov
2021-02-24  2:30         ` Xiaoguang Wang
2021-02-24  2:35           ` Jens Axboe
2021-02-24  2:45             ` Xiaoguang Wang
2021-02-24  2:51               ` Jens Axboe
2021-02-24  9:46                 ` Pavel Begunkov
2021-02-24  9:59                   ` Pavel Begunkov
2021-02-24 10:33                     ` Pavel Begunkov
2021-02-24  9:38           ` Pavel Begunkov
2021-02-24 12:42   ` Hao Xu
2021-02-25 10:55     ` Pavel Begunkov
2021-02-24  3:23 ` Xiaoguang Wang

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