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From: Wouter Verhelst <w@uter.be>
To: Hou Tao <houtao1@huawei.com>
Cc: Christoph Hellwig <hch@lst.de>,
	Josef Bacik <josef@toxicpanda.com>, Jens Axboe <axboe@kernel.dk>,
	linux-block@vger.kernel.org, nbd@other.debian.org
Subject: Re: [PATCH v2 3/3] nbd: fix race between nbd_alloc_config() and module removal
Date: Tue, 14 Sep 2021 13:42:27 +0200	[thread overview]
Message-ID: <YUCKoySdM9WZlUH9@pc181009.grep.be> (raw)
In-Reply-To: <6434d4e8-984d-97df-34e5-b86a0e69cf58@huawei.com>

On Mon, Sep 13, 2021 at 12:32:37PM +0800, Hou Tao wrote:
> Hi Christoph,
> 
> On 9/9/2021 2:40 PM, Christoph Hellwig wrote:
> > On Tue, Sep 07, 2021 at 11:04:16AM +0800, Hou Tao wrote:
> >> Let me explain first. The reason it works is due to genl_lock_all() in netlink code.
> > Btw, please properly format your mail.  These overly long lines are really
> > hard to read.
> Thanks for reminding.
> >> If the module removal happens before calling try_module_get(), nbd_cleanup() will
> >>
> >> call genl_unregister_family() first, and then genl_lock_all(). genl_lock_all() will
> >>
> >> prevent ops in nbd_connect_genl_ops() from being called, because the calling
> >>
> >> of nbd ops happens in genl_rcv() which needs to acquire cb_lock first.
> > Good.
> >
> >> I have checked multiple genl_ops, it seems that the reason why these genl_ops
> >>
> >> don't need try_module_get() is that these ops don't create new object through
> >>
> >> genl_ops and just control it. However genl_family_rcv_msg_dumpit() will try to
> >>
> >> call try_module_get(), but according to the history (6dc878a8ca39 "netlink: add reference of module in netlink_dump_start"),
> >>
> >> it is because inet_diag_handler_cmd() will call __netlink_dump_start().
> > And now taking a step back:  Why do we even need this extra module
> > reference?  For the case where nbd_alloc_config is called from nbd_open
> > we obviously don't need it.  In the case where it is called from
> > nbd_genl_connect that prevents unloading nbd when there is a configured
> > but not actually nbd device.  Which isn't reallyed need and counter to
> > how other drivers work.
> Yes, the purpose of module ref-counting in nbd_alloc_config() is to force
> the user to disconnect the nbd device manually before module removal.
> And loop device works in the same way. If a file is attached to a loop device,
> an extra module reference will be taken in loop_configure() and the removal
> of loop module will fail. The only difference is that loop driver takes the
> extra ref-count by ioctl, and nbd does it through netlink.

Haven't checked the actual patch, but just wanted to point out:

nbd should do it through netlink *and* ioctl -- the older way to
configure nbd was through ioctl, which we should still support for
backcompat reasons.

(if that's already the case, then ignore what I said :-)

-- 
     w@uter.{be,co.za}
wouter@{grep.be,fosdem.org,debian.org}

      parent reply	other threads:[~2021-09-14 11:42 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-09-04 12:25 [PATCH v2 0/3] fix races between nbd setup and module removal Hou Tao
2021-09-04 12:25 ` [PATCH v2 1/3] nbd: use pr_err to output error message Hou Tao
2021-09-06  9:27   ` Christoph Hellwig
2021-09-04 12:25 ` [PATCH v2 2/3] nbd: call genl_unregister_family() first in nbd_cleanup() Hou Tao
2021-09-06  9:27   ` Christoph Hellwig
2021-09-04 12:25 ` [PATCH v2 3/3] nbd: fix race between nbd_alloc_config() and module removal Hou Tao
2021-09-06  9:30   ` Christoph Hellwig
2021-09-06 10:08     ` Hou Tao
2021-09-06 10:25       ` Christoph Hellwig
2021-09-07  3:04         ` Hou Tao
2021-09-08 13:03           ` Hou Tao
2021-09-09  6:40           ` Christoph Hellwig
2021-09-13  4:32             ` Hou Tao
2021-09-13 15:25               ` Christoph Hellwig
2021-09-14 11:42               ` Wouter Verhelst [this message]

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