netdev - Re: [Patch net] rds: mark bound socket with SOCK_RCU

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Message-ID: <CAM_iQpVt4hOs8bYFVm7mncCoTVqo6HZGgRAxyiO5t=Z+Pq+QrQ@mail.gmail.com>
Date:   Mon, 10 Sep 2018 17:39:03 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Sowmini Varadhan <sowmini.varadhan@...cle.com>
Cc:     Santosh Shilimkar <santosh.shilimkar@...cle.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        rds-devel@....oracle.com
Subject: Re: [Patch net] rds: mark bound socket with SOCK_RCU_FREE

On Mon, Sep 10, 2018 at 5:24 PM Sowmini Varadhan
<sowmini.varadhan@...cle.com> wrote:
>
> On (09/10/18 17:16), Cong Wang wrote:
> > >
> > > On (09/10/18 16:51), Cong Wang wrote:
> > > >
> > > >         __rds_create_bind_key(key, addr, port, scope_id);
> > > > -       rs = rhashtable_lookup_fast(&bind_hash_table, key, ht_parms);
> > > > +       rcu_read_lock();
> > > > +       rs = rhashtable_lookup(&bind_hash_table, key, ht_parms);
> > > >         if (rs && !sock_flag(rds_rs_to_sk(rs), SOCK_DEAD))
> > > >                 rds_sock_addref(rs);
> > > >         else
> > > >                 rs = NULL;
> > > > +       rcu_read_unlock();
> > >
> > > aiui, the rcu_read lock/unlock is only useful if the write
> > > side doing destructive operations  does something to make sure readers
> > > are done before doing the destructive opertion. AFAIK, that does
> > > not exist for rds socket management today
> >
> > That is exactly why we need it here, right?
>
> Maybe I am confused, what exactly is the patch you are proposing?
>
> Does it have the SOCK_RCU_FREE change?

Yes, that patch is obviously on top of this patch.


> Does it have the rcu_read_lock you have above?
> Where is the call_rcu?

Sure, as it is on top of this patch, the call_rcu() is
here:

void sk_destruct(struct sock *sk)
{
        if (sock_flag(sk, SOCK_RCU_FREE))
                call_rcu(&sk->sk_rcu, __sk_destruct);
        else
                __sk_destruct(&sk->sk_rcu);
}


>
> > Hmm, so you are saying synchronize_rcu() is kinda more correct
> > than call_rcu()??
>
>
> I'm not saying that, I'm asking "what exactly is the patch
> you are proposing?" The only one on record is
>    http://patchwork.ozlabs.org/patch/968282/
> which does not have either synchronize_rcu or call_rcu.

It is very obviously on top of this patch ($subject).


>
> > I never hear this before, would like to know why.
>
> Please post precise patches first.

Already showed you precisely what is is, just on top
of this one.