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

[Neil Horman <nhorman@tuxdriver.com>]

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 & Regards
Neil

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 *Neil Horman
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 *Red Hat, Inc.
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