linux-kernel - Re: [PATCH 0/2] Fix (improve) deadlock condition on module removal netfilter socket option removal

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Message-ID: <20070906110836.GA31868@hmsreliant.think-freely.org>
Date:	Thu, 6 Sep 2007 07:08:36 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Patrick McHardy <kaber@...sh.net>
Cc:	Rusty Russell <rusty@...tcorp.com.au>, adam@...drasil.com,
	jcm@...masters.org, netfilter-devel@...ts.netfilter.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/2] Fix (improve) deadlock condition on module removal netfilter socket option removal

On Thu, Sep 06, 2007 at 12:33:52PM +0200, Patrick McHardy wrote:
> Neil Horman wrote:
> > On Thu, Sep 06, 2007 at 02:13:26AM +1000, Rusty Russell wrote:
> > 
> >>On Wed, 2007-09-05 at 17:22 +0200, Patrick McHardy wrote:
> >>
> >>>But I'm wondering, wouldn't module refcounting alone fix this problem?
> >>>If we make nf_sockopt() call try_module_get(ops->owner), remove_module()
> >>>on ip_tables.ko would simply fail because the refcount is above zero
> >>>(so it would fail at point 3 above). Am I missing something important?
> >>
> >>Yes, that seems the correct solution to me, too.  ISTR that this code
> >>predates the current module code.
> >>
> >>Rusty.
> > 
> > 
> > Thanks guys-
> > 	When I first started looking at this problem I would have agreed with
> > you, that module reference counting alone would fix the problem.  However,
> > delete_module can work in either a non-blocking or a blocking mode.  rmmod
> > passes O_NONBLOCK to delete module, and so is fine, but modprobe does not.  So
> > if you currently use modprobe -r to remove modules (as the iptables service
> > script nominally does), modprobe winds up waiting in the kernel for the module
> > reference count to become zero.  Since we can hold a reference to the module
> > being removed in the same path that forks a modprobe request to load that same
> > module (which then blocks on the first modprobes fcntl lock), we still get
> > deadlock.  The way I fixed this was by use of the second patch, which brings
> > modprobes behavior into line with the rmmod utility (which is to default to
> > non-blocking operation), leading to the remove_module failure and breaking of
> > the deadlock that you describe above.
> 
> 
> Thanks for the explanation, I've applied your patch.
Thanks Patrick!
Neil
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 *Neil Horman
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 *Red Hat, Inc.
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