Re: [PATCH V3] io_uring: fix IO hang in io_wq_put_and_exit from do_exit()

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Sep 15, 2023 at 03:04:05PM +0800, Jason Wang wrote:
> On Fri, Sep 8, 2023 at 11:25 PM Ming Lei <ming.lei@redhat.com> wrote:
> >
> > On Fri, Sep 08, 2023 at 08:44:45AM -0600, Jens Axboe wrote:
> > > On 9/8/23 8:34 AM, Ming Lei wrote:
> > > > On Fri, Sep 08, 2023 at 07:49:53AM -0600, Jens Axboe wrote:
> > > >> On 9/8/23 3:30 AM, Ming Lei wrote:
> > > >>> diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
> > > >>> index ad636954abae..95a3d31a1ef1 100644
> > > >>> --- a/io_uring/io_uring.c
> > > >>> +++ b/io_uring/io_uring.c
> > > >>> @@ -1930,6 +1930,10 @@ void io_wq_submit_work(struct io_wq_work *work)
> > > >>>           }
> > > >>>   }
> > > >>>
> > > >>> + /* It is fragile to block POLLED IO, so switch to NON_BLOCK */
> > > >>> + if ((req->ctx->flags & IORING_SETUP_IOPOLL) && def->iopoll_queue)
> > > >>> +         issue_flags |= IO_URING_F_NONBLOCK;
> > > >>> +
> > > >>
> > > >> I think this comment deserves to be more descriptive. Normally we
> > > >> absolutely cannot block for polled IO, it's only OK here because io-wq
> > > >
> > > > Yeah, we don't do that until commit 2bc057692599 ("block: don't make REQ_POLLED
> > > > imply REQ_NOWAIT") which actually push the responsibility/risk up to
> > > > io_uring.
> > > >
> > > >> is the issuer and not necessarily the poller of it. That generally falls
> > > >> upon the original issuer to poll these requests.
> > > >>
> > > >> I think this should be a separate commit, coming before the main fix
> > > >> which is below.
> > > >
> > > > Looks fine, actually IO_URING_F_NONBLOCK change isn't a must, and the
> > > > approach in V2 doesn't need this change.
> > > >
> > > >>
> > > >>> @@ -3363,6 +3367,12 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
> > > >>>           finish_wait(&tctx->wait, &wait);
> > > >>>   } while (1);
> > > >>>
> > > >>> + /*
> > > >>> +  * Reap events from each ctx, otherwise these requests may take
> > > >>> +  * resources and prevent other contexts from being moved on.
> > > >>> +  */
> > > >>> + xa_for_each(&tctx->xa, index, node)
> > > >>> +         io_iopoll_try_reap_events(node->ctx);
> > > >>
> > > >> The main issue here is that if someone isn't polling for them, then we
> > > >
> > > > That is actually what this patch is addressing, :-)
> > >
> > > Right, that part is obvious :)
> > >
> > > >> get to wait for a timeout before they complete. This can delay exit, for
> > > >> example, as we're now just waiting 30 seconds (or whatever the timeout
> > > >> is on the underlying device) for them to get timed out before exit can
> > > >> finish.
> > > >
> > > > For the issue on null_blk, device timeout handler provides
> > > > forward-progress, such as requests are released, so new IO can be
> > > > handled.
> > > >
> > > > However, not all devices support timeout, such as virtio device.
> > >
> > > That's a bug in the driver, you cannot sanely support polled IO and not
> > > be able to deal with timeouts. Someone HAS to reap the requests and
> > > there are only two things that can do that - the application doing the
> > > polled IO, or if that doesn't happen, a timeout.
> >
> > OK, then device driver timeout handler has new responsibility of covering
> > userspace accident, :-)

Sorry, I don't have enough context so this is probably a silly question:

When an application doesn't reap a polled request, why doesn't the block
layer take care of this in a generic way? I don't see anything
driver-specific about this.

Driver-specific behavior would be sending an abort/cancel upon timeout.
virtio-blk cannot do that because there is no such command in the device
specification at the moment. So simply waiting for the polled request to
complete is the only thing that can be done (aside from resetting the
device), and it's generic behavior.

Thanks,
Stefan

> >
> > We may document this requirement for driver.
> >
> > So far the only one should be virtio-blk, and the two virtio storage
> > drivers never implement timeout handler.
> >
> 
> Adding Stefan for more comments.
> 
> Thanks
> 

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