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Date:   Mon, 24 Aug 2020 14:39:35 +0200
From:   Florian Westphal <fw@strlen.de>
To:     Pablo Neira Ayuso <pablo@netfilter.org>
Cc:     Florian Westphal <fw@strlen.de>, Phil Sutter <phil@nwl.cc>,
        netfilter-devel@vger.kernel.org
Subject: Re: nfnetlink: Busy-loop in nfnetlink_rcv_msg()
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Pablo Neira Ayuso <pablo@netfilter.org> wrote:
> > We can of course also intercept -EAGAIN in nf_tables_api.c and translate
> > it to -ENOBUFS like in nft_get_set_elem().
> > 
> > But I think a generic solution it better.  The call_rcu backends should
> > not result in changes to nf_tables internal state so they do not load
> > modules and therefore don't need a restart.
> 
> Handling this from the core would be better, so people don't have to
> remember to use the nfnetlink_unicast() that I'm proposing.

Your patch looks good to me.

> Looking at the tree, call_rcu is not enough to assume this: there are
> several nfnetlink subsystems calling netlink_unicast() that translate
> EAGAIN to ENOBUFS, from .call and .call_rcu. The way to identify this
> would be to decorate callbacks to know what are specifically GET
> commands.

?  Which .call_rcu is NOT "passive" (does not just peek at things) and,
more specifically, which .call_rcu depends on -EAGAIN requiring a
tansaction replay?