linux-kernel - Re: [PATCH v2] nbd: Don't use workqueue to handle recv work

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Date:   Thu, 30 Dec 2021 12:01:23 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Christoph Hellwig <hch@...radead.org>
Cc:     Josef Bacik <josef@...icpanda.com>, Jens Axboe <axboe@...nel.dk>,
        Bart Van Assche <bvanassche@....org>,
        linux-block@...r.kernel.org, nbd@...er.debian.org,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] nbd: Don't use workqueue to handle recv work

On Thu, Dec 30, 2021 at 1:35 AM Christoph Hellwig <hch@...radead.org> wrote:
>
> On Mon, Dec 27, 2021 at 05:12:41PM +0800, Xie Yongji wrote:
> > The rescuer thread might take over the works queued on
> > the workqueue when the worker thread creation timed out.
> > If this happens, we have no chance to create multiple
> > recv threads which causes I/O hung on this nbd device.
>
> If a workqueue is used there aren't really 'receive threads'.
> What is the deadlock here?

We might have multiple recv works, and those recv works won't quit
unless the socket is closed. If the rescuer thread takes over those
works, only the first recv work can run. The I/O needed to be handled
in other recv works would be hung since no thread can handle them.

In that case, we can see below stacks in rescuer thread:

__schedule
  schedule
    scheule_timeout
      unix_stream_read_generic
        unix_stream_recvmsg
          sock_xmit
            nbd_read_stat
              recv_work
                process_one_work
                  rescuer_thread
                    kthread
                      ret_from_fork

Thanks,
Yongji