netdev - Re: [PATCH net 2/2] auth_gss: Fix deadlock that blocks rpcsec_gss_exit

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Message-ID: <20210928134952.GA25415@fieldses.org>
Date:   Tue, 28 Sep 2021 09:49:52 -0400
From:   "bfields@...ldses.org" <bfields@...ldses.org>
To:     Trond Myklebust <trondmy@...merspace.com>
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Subject: Re: [PATCH net 2/2] auth_gss: Fix deadlock that blocks
 rpcsec_gss_exit_net when use-gss-proxy==1

On Tue, Sep 28, 2021 at 01:30:17PM +0000, Trond Myklebust wrote:
> On Tue, 2021-09-28 at 11:14 +0800, Wang Hai wrote:
> > When use-gss-proxy is set to 1, write_gssp() creates a rpc client in
> > gssp_rpc_create(), this increases the netns refcount by 2, these
> > refcounts are supposed to be released in rpcsec_gss_exit_net(), but
> > it
> > will never happen because rpcsec_gss_exit_net() is triggered only
> > when
> > the netns refcount gets to 0, specifically:
> >     refcount=0 -> cleanup_net() -> ops_exit_list ->
> > rpcsec_gss_exit_net
> > It is a deadlock situation here, refcount will never get to 0 unless
> > rpcsec_gss_exit_net() is called. So, in this case, the netns refcount
> > should not be increased.
> > 
> > In this case, xprt will take a netns refcount which is not supposed
> > to be taken. Add a new flag to rpc_create_args called
> > RPC_CLNT_CREATE_NO_NET_REF for not increasing the netns refcount.
> > 
> > It is safe not to hold the netns refcount, because when
> > cleanup_net(), it
> > will hold the gssp_lock and then shut down the rpc client
> > synchronously.
> > 
> > 
> I don't like this solution at all. Adding this kind of flag is going to
> lead to problems down the road.
> 
> Is there any reason whatsoever why we need this RPC client to exist
> when there is no active knfsd server? IOW: Is there any reason why we
> shouldn't defer creating this RPC client for when knfsd starts up in
> this net namespace, and why we can't shut it down when knfsd shuts
> down?

The rpc create is done in the context of the process that writes to
/proc/net/rpc/use-gss-proxy to get the right namespaces.  I don't know
how hard it would be capture that information for a later create.

--b.