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

Yongji Xie
2021-12-30T04:01:38Z

On Thu, Dec 30, 2021 at 1:35 AM Christoph Hellwig <hch@infradead.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