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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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    *Red Hat, Inc.
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