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

[Pavel Begunkov <asml.silence@gmail.com>]

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