Re: [PATCH 4/4] io_uring: cancel sqpoll via task_work

[Jens Axboe <axboe@kernel.dk>]

On 3/12/21 12:35 PM, Pavel Begunkov wrote:
> On 11/03/2021 23:29, Pavel Begunkov wrote:
>> 1) The first problem is io_uring_cancel_sqpoll() ->
>> io_uring_cancel_task_requests() basically doing park(); park(); and so
>> hanging.
>>
>> 2) Another one is more subtle, when the master task is doing cancellations,
>> but SQPOLL task submits in-between the end of the cancellation but
>> before finish() requests taking a ref to the ctx, and so eternally
>> locking it up.
>>
>> 3) Yet another is a dying SQPOLL task doing io_uring_cancel_sqpoll() and
>> same io_uring_cancel_sqpoll() from the owner task, they race for
>> tctx->wait events. And there probably more of them.
>>
>> Instead do SQPOLL cancellations from within SQPOLL task context via
>> task_work, see io_sqpoll_cancel_sync(). With that we don't need temporal
>> park()/unpark() during cancellation, which is ugly, subtle and anyway
>> doesn't allow to do io_run_task_work() properly.> 
>> io_uring_cancel_sqpoll() is called only from SQPOLL task context and
>> under sqd locking, so all parking is removed from there. And so,
>> io_sq_thread_[un]park() and io_sq_thread_stop() are not used now by
>> SQPOLL task, and that spare us from some headache.
>>
>> Also remove ctx->sqd_list early to avoid 2). And kill tctx->sqpoll,
>> which is not used anymore.
> 
> 
> Looks, the chunk below somehow slipped from the patch. Not important
> for 5.12, but can can be folded anyway
> 
> diff --git a/include/linux/io_uring.h b/include/linux/io_uring.h
> index 9761a0ec9f95..c24c62b47745 100644
> --- a/include/linux/io_uring.h
> +++ b/include/linux/io_uring.h
> @@ -22,7 +22,6 @@ struct io_uring_task {
>  	void			*io_wq;
>  	struct percpu_counter	inflight;
>  	atomic_t		in_idle;
> -	bool			sqpoll;
>  
>  	spinlock_t		task_lock;
>  	struct io_wq_work_list	task_list;

Let's do it as a separate patch instead.

-- 
Jens Axboe