Re: [PATCH] io_uring: complete request via task work in case of DEFER_TASKRUN

[Kanchan Joshi <joshi.k@samsung.com>]

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On Fri, Apr 14, 2023 at 09:53:15PM +0800, Ming Lei wrote:
>On Fri, Apr 14, 2023 at 02:01:26PM +0100, Pavel Begunkov wrote:
>> On 4/14/23 08:53, Ming Lei wrote:
>> > So far io_req_complete_post() only covers DEFER_TASKRUN by completing
>> > request via task work when the request is completed from IOWQ.
>> >
>> > However, uring command could be completed from any context, and if io
>> > uring is setup with DEFER_TASKRUN, the command is required to be
>> > completed from current context, otherwise wait on IORING_ENTER_GETEVENTS
>> > can't be wakeup, and may hang forever.
>>
>> fwiw, there is one legit exception, when the task is half dead
>> task_work will be executed by a kthread. It should be fine as it
>> locks the ctx down, but I can't help but wonder whether it's only
>> ublk_cancel_queue() affected or there are more places in ublk?
>
>No, it isn't.
>
>It isn't triggered on nvme-pt just because command is always done
>in task context.
>
>And we know more uring command cases are coming.

FWIW, the model I had in mind (in initial days) was this -
1) io_uring_cmd_done is a simple API, it just posts one/two results into
reuglar/big SQE
2) for anything complex completion (that requires task-work), it will
use another API io_uring_cmd_complete_in_task with the provider-specific
callback (that will call above simple API eventually)


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