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SubjectRe: [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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