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From: Christoph Hellwig <hch@lst.de>
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: Mon, 6 Sep 2021 12:25:21 +0200	[thread overview]
Message-ID: <20210906102521.GA3082@lst.de> (raw)
In-Reply-To: <ce3e1ea8-ebda-4372-42ce-e8a4b2d12514@huawei.com>

On Mon, Sep 06, 2021 at 06:08:54PM +0800, Hou Tao wrote:
> >> +	if (!try_module_get(THIS_MODULE))
> >> +		return ERR_PTR(-ENODEV);
> > try_module_get(THIS_MODULE) is an indicator for an unsafe pattern.  If
> > we don't already have a reference it could never close the race.
> >
> > Looking at the callers:
> >
> >  - nbd_open like all block device operations must have a reference
> >    already.
> Yes. nbd_open() has already taken a reference in dentry_open().
> >  - for nbd_genl_connect I'm not an expert, but given that struct
> >    nbd_genl_family has a module member I suspect the networkinh
> >    code already takes a reference.
> 
> That was my original though, but the fact is netlink code doesn't take a module reference
> 
> in genl_family_rcv_msg_doit() and netlink uses genl_lock_all() to serialize between module removal
> 
> and nbd_connect_genl_ops calling, so I think use try_module_get() is OK here.

How it this going to work?  If there was a race you just shortened it,
but it can still happen before you call try_module_get.  So I think we
need to look into how the netlink calling conventions are supposed to
look and understand the issues there first.

  reply	other threads:[~2021-09-06 10:25 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 [this message]
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

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