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

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Date:   Mon, 10 Sep 2018 20:24:23 -0400
From:   Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:     Cong Wang <xiyou.wangcong@...il.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 (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? 
Does it have the rcu_read_lock you have above? 
Where is the call_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.

> I never hear this before, would like to know why.

Please post precise patches first.

Thanks.