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From: Ming Lei <[email protected]>
To: Jens Axboe <[email protected]>
Cc: [email protected], [email protected],
	David Howells <[email protected]>,
	Chengming Zhou <[email protected]>,
	[email protected]
Subject: Re: [PATCH V2] io_uring: fix IO hang in io_wq_put_and_exit from do_exit()
Date: Fri, 1 Sep 2023 23:12:00 +0800	[thread overview]
Message-ID: <ZPH/QEXmh3NiY736@fedora> (raw)
In-Reply-To: <[email protected]>

On Fri, Sep 01, 2023 at 08:47:28AM -0600, Jens Axboe wrote:
> On 9/1/23 7:49 AM, Ming Lei wrote:
> > io_wq_put_and_exit() is called from do_exit(), but all FIXED_FILE requests
> > in io_wq aren't canceled in io_uring_cancel_generic() called from do_exit().
> > Meantime io_wq IO code path may share resource with normal iopoll code
> > path.
> > 
> > So if any HIPRI request is submittd via io_wq, this request may not get resouce
> > for moving on, given iopoll isn't possible in io_wq_put_and_exit().
> > 
> > The issue can be triggered when terminating 't/io_uring -n4 /dev/nullb0'
> > with default null_blk parameters.
> > 
> > Fix it by always cancelling all requests in io_wq by adding helper of
> > io_uring_cancel_wq(), and this way is reasonable because io_wq destroying
> > follows canceling requests immediately.
> 
> This does look much cleaner, but the unconditional cancel_all == true
> makes me a bit nervous in case the ring is being shared.

Here we just cancel requests in io_wq, which is per-task actually.

Yeah, ctx->iopoll_ctx could be shared, but if it is used in this way,
the event can't be avoided to reap from remote context.

> 
> Do we really need to cancel these bits? Can't we get by with something
> trivial like just stopping retrying if the original task is exiting?
> 
> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> index c6d9e4677073..95316c0c3830 100644
> --- a/io_uring/io_uring.c
> +++ b/io_uring/io_uring.c
> @@ -1939,7 +1939,7 @@ void io_wq_submit_work(struct io_wq_work *work)
>  		 * If REQ_F_NOWAIT is set, then don't wait or retry with
>  		 * poll. -EAGAIN is final for that case.
>  		 */
> -		if (req->flags & REQ_F_NOWAIT)
> +		if (req->flags & REQ_F_NOWAIT || req->task->flags & PF_EXITING)
>  			break;

This way isn't enough, any request submitted to io_wq before do_exit()
need to be reaped by io_iopoll_try_reap_events() explicitly.

Not mention IO_URING_F_NONBLOCK isn't set, so io_issue_sqe() may hang
forever.


Thanks,
Ming


  reply	other threads:[~2023-09-01 15:13 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2023-09-01 13:49 [PATCH V2] io_uring: fix IO hang in io_wq_put_and_exit from do_exit() Ming Lei
2023-09-01 14:47 ` Jens Axboe
2023-09-01 15:12   ` Ming Lei [this message]
2023-09-01 15:13     ` Jens Axboe
2023-09-01 15:13 ` Jens Axboe
2023-09-07 15:36   ` Jens Axboe
2023-09-08  1:03     ` Pavel Begunkov
2023-09-08  6:22       ` Ming Lei
2023-09-08 16:01         ` Pavel Begunkov

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