netdev - Re: [PATCH v2 net] inet: frags: rework rhashtable dismantle

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Date:   Tue, 2 Oct 2018 06:16:27 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Thomas Graf <tgraf@...g.ch>,
        Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [PATCH v2 net] inet: frags: rework rhashtable dismantle

On Tue, Oct 2, 2018 at 1:19 AM Dmitry Vyukov <dvyukov@...gle.com> wrote:
>
> On Tue, Oct 2, 2018 at 7:49 AM, Eric Dumazet <edumazet@...gle.com> wrote:
>
>
> Does inet_frag_kill() hold fq->lock? I am missing how inet_frag_kill()
> and inet_frags_exit_net() are synchronized.
> Since you use smp_store_release()/READ_ONCE() they seem to run in
> parallel. But then isn't it possible that inet_frag_kill() reads
> nf->dead == 0, then inet_frags_exit_net() sets nf->dead, and then we
> have the same race on concurrent removal? Or, isn't it possible that
> inet_frag_kill() reads nf->dead == 1, but does not set
> INET_FRAG_HASH_DEAD yet, and then inet_frags_free_cb() misses the
> INET_FRAG_HASH_DEAD flag?
>

Yes this is kind of implied in my patch.
I put the smp_store_release() and READ_ONCE exactly to document the
possible races.
This was the reason for my attempt in V1, doing a walk, but Herbert
said walk was not designed for doing deletes.

Proper synch will need a synchronize_rcu(), and thus a future
conversion in net-next because we can not really
add new synchronize_rcu() calls in an (struct
pernet_operations.)exit() without considerable performance hit of
netns dismantles.

So this will require a conversion of all inet_frags_exit_net() callers
to .exit_batch() to mitigate the cost.

I thought of synchronize_rcu_bh() but this beast is going away soon anyway.