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

[Jens Axboe <axboe@kernel.dk>]

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.

> Here we just call io_iopoll_try_reap_events() to poll submitted IOs
> for releasing resources, so no need to rely on device timeout handler
> any more, and the extra exit delay can be avoided.
> 
> But io_iopoll_try_reap_events() may not be enough because io_wq
> associated with current context can get released resource immediately,
> then new IOs are submitted successfully, but who can poll these new
> submitted IOs, then all device resources can be held by this (freed)io_wq
> for nothing.
> 
> I guess we may have to take the approach in patch V2 by only canceling
> polled IO for avoiding the thread_exit regression, or other ideas?

Ideally the behavior seems like it should be that if a task goes away,
any pending polled IO it has should be reaped. With the above notion
that a driver supporting poll absolutely must be able to deal with
timeouts, it's not a strict requirement as we know that requests will be
reaped.

>> Do we just want to move this a bit higher up where we iterate ctx's
>> anyway? Not that important I suspect.
> 
> I think it isn't needed, here we only focus on io_wq and polled io,
> not same with what the iteration code covers, otherwise
> io_uring_try_cancel_requests could become less readable.

Yeah, this part isn't a big deal at all, more of a stylistic thing.

-- 
Jens Axboe