Re: [PATCH V2] io_uring: fix IO hang in io_wq_put_and_exit from do_exit()

[Ming Lei <ming.lei@redhat.com>]

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