From: Yongji Xie <xieyongji@bytedance.com>
To: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@infradead.org>,
Jens Axboe <axboe@kernel.dk>,
Bart Van Assche <bvanassche@acm.org>,
linux-block@vger.kernel.org, nbd@other.debian.org,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2] nbd: Don't use workqueue to handle recv work
Date: Tue, 4 Jan 2022 13:31:47 +0800 [thread overview]
Message-ID: <CACycT3vYt0XNV2GdjKjDS1iyWieY_OV4h=W1qqk_AAAahRZowA@mail.gmail.com> (raw)
In-Reply-To: <YdMgCS1RMcb5V2RJ@localhost.localdomain>
On Tue, Jan 4, 2022 at 12:10 AM Josef Bacik <josef@toxicpanda.com> wrote:
>
> On Thu, Dec 30, 2021 at 12:01:23PM +0800, Yongji Xie wrote:
> > 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.
> >
>
> I'm not following this explanation. What is the rescuer thread you're talking
https://www.kernel.org/doc/html/latest/core-api/workqueue.html#c.rescuer_thread
> about? If there's an error we close the socket which will error out the recvmsg
> which will make the recv workqueue close down.
When to close the socket? The nbd daemon doesn't know what happens in
the kernel.
>
> > 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
>
> This is just the thing hanging waiting for an incoming request, so this doesn't
> tell me anything. Thanks,
>
The point is the *recv_work* is handled in the *rescuer_thread*.
Normally it should be handled in *work_thread* like:
__schedule
schedule
scheule_timeout
unix_stream_read_generic
unix_stream_recvmsg
sock_xmit
nbd_read_stat
recv_work
process_one_work
*work_thread*
kthread
ret_from_fork
Thanks,
Yongji
next prev parent reply other threads:[~2022-01-04 5:32 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-27 9:12 [PATCH v2] nbd: Don't use workqueue to handle recv work Xie Yongji
2021-12-29 17:35 ` Christoph Hellwig
2021-12-30 4:01 ` Yongji Xie
2022-01-03 16:10 ` Josef Bacik
2022-01-04 5:31 ` Yongji Xie [this message]
2022-01-04 18:06 ` Josef Bacik
2022-01-05 5:36 ` Yongji Xie
2022-01-21 8:34 ` Yongji Xie
2022-02-15 13:17 ` Yongji Xie
2022-02-18 15:20 ` Josef Bacik
2022-02-19 13:04 ` Yongji Xie
2022-03-22 8:10 ` Yongji Xie
2022-03-22 20:11 ` Josef Bacik
2022-03-23 11:21 ` Yongji Xie
2022-05-16 6:13 ` Yongji Xie
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