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From: Hou Tao <>
To: Christoph Hellwig <>
Cc: Josef Bacik <>, Jens Axboe <>,
	<>, <>
Subject: Re: [PATCH v2 3/3] nbd: fix race between nbd_alloc_config() and module removal
Date: Mon, 13 Sep 2021 12:32:37 +0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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.
> Did you try just removing the extra module refcounting?
Yes, removing the module refcounting in nbd_alloc_config() and cleaning
the nbd_config in nbd_cleanup() also work, but not sure whether or not
it will break some nbd user-case which depends on the extra module
reference count. I prefer to keep the extra module refcounting considering
that loop driver does it as well, so what is your suggestion ?


> .

  reply	other threads:[~2021-09-13  4:32 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 [this message]
2021-09-13 15:25               ` Christoph Hellwig
2021-09-14 11:42               ` Wouter Verhelst

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