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

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On 9/9/23 02:43, Ming Lei wrote:
On Fri, Sep 08, 2023 at 04:46:15PM +0100, Pavel Begunkov wrote:
On 9/8/23 14:49, 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
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.

@@ -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
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

Ok, our case is that userspace crashes and doesn't poll for its IO.
How would that block io-wq termination? We send a signal and workers
should exit, either by queueing up the request for iopoll (and then

It depends on how userspace handles the signal, such as, t/io_uring,
s->finish is set as true in INT signal handler, two cases may happen:

1) s->finish is observed immediately, then this pthread exits, and leave
polled requests in ctx->iopoll_list

fwiw, I'm in favour of trying to iopoll there just because it's nicer
this way, but I still want to get to the bottom of it.

2) s->finish isn't observed immediately, and just submit & polling;
if any IO can't be submitted because of no enough resource, there can
be one busy spin because submitter_uring_fn() waits for inflight IO.

So if there are two pthreads(A, B), each setup its own io_uring context
and submit & poll IO on same block device.  If 1) happens in A, all
device tags can be held for nothing.  If 2) happens in B, the busy spin
prevents exit() of this pthread B.

Thanks, that sounds clear now. So, nobody closes the first ring, hence
it's not destroyed even after pthread A exits and the 2nd ring cannot
progress. I agree with the judgement about timeouts and that it looks
like a user mismanagement.

Pavel Begunkov

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