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Date:   Mon, 7 Dec 2020 12:14:38 -0800
From:   Martin KaFai Lau <kafai@fb.com>
To:     Kuniyuki Iwashima <kuniyu@amazon.co.jp>
CC:     <ast@kernel.org>, <benh@amazon.com>, <bpf@vger.kernel.org>,
        <daniel@iogearbox.net>, <davem@davemloft.net>,
        <edumazet@google.com>, <kuba@kernel.org>, <kuni1840@gmail.com>,
        <linux-kernel@vger.kernel.org>, <netdev@vger.kernel.org>
Subject: Re: [PATCH v1 bpf-next 03/11] tcp: Migrate
 TCP_ESTABLISHED/TCP_SYN_RECV sockets in accept queues.
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On Sun, Dec 06, 2020 at 01:03:07AM +0900, Kuniyuki Iwashima wrote:
> From:   Martin KaFai Lau <kafai@fb.com>
> Date:   Fri, 4 Dec 2020 17:42:41 -0800
> > On Tue, Dec 01, 2020 at 11:44:10PM +0900, Kuniyuki Iwashima wrote:
> > [ ... ]
> > > diff --git a/net/core/sock_reuseport.c b/net/core/sock_reuseport.c
> > > index fd133516ac0e..60d7c1f28809 100644
> > > --- a/net/core/sock_reuseport.c
> > > +++ b/net/core/sock_reuseport.c
> > > @@ -216,9 +216,11 @@ int reuseport_add_sock(struct sock *sk, struct sock *sk2, bool bind_inany)
> > >  }
> > >  EXPORT_SYMBOL(reuseport_add_sock);
> > >  
> > > -void reuseport_detach_sock(struct sock *sk)
> > > +struct sock *reuseport_detach_sock(struct sock *sk)
> > >  {
> > >  	struct sock_reuseport *reuse;
> > > +	struct bpf_prog *prog;
> > > +	struct sock *nsk = NULL;
> > >  	int i;
> > >  
> > >  	spin_lock_bh(&reuseport_lock);
> > > @@ -242,8 +244,12 @@ void reuseport_detach_sock(struct sock *sk)
> > >  
> > >  		reuse->num_socks--;
> > >  		reuse->socks[i] = reuse->socks[reuse->num_socks];
> > > +		prog = rcu_dereference(reuse->prog);
> > Is it under rcu_read_lock() here?
> 
> reuseport_lock is locked in this function, and we do not modify the prog,
> but is rcu_dereference_protected() preferable?
> 
> ---8<---
> prog = rcu_dereference_protected(reuse->prog,
> 				 lockdep_is_held(&reuseport_lock));
> ---8<---
It is not only reuse->prog.  Other things also require rcu_read_lock(),
e.g. please take a look at __htab_map_lookup_elem().

The TCP_LISTEN sk (selected by bpf to be the target of the migration)
is also protected by rcu.

I am surprised there is no WARNING in the test.
Do you have the needed DEBUG_LOCK* config enabled?

> > >  		if (sk->sk_protocol == IPPROTO_TCP) {
> > > +			if (reuse->num_socks && !prog)
> > > +				nsk = i == reuse->num_socks ? reuse->socks[i - 1] : reuse->socks[i];
> > > +
> > >  			reuse->num_closed_socks++;
> > >  			reuse->socks[reuse->max_socks - reuse->num_closed_socks] = sk;
> > >  		} else {
> > > @@ -264,6 +270,8 @@ void reuseport_detach_sock(struct sock *sk)
> > >  		call_rcu(&reuse->rcu, reuseport_free_rcu);
> > >  out:
> > >  	spin_unlock_bh(&reuseport_lock);
> > > +
> > > +	return nsk;
> > >  }
> > >  EXPORT_SYMBOL(reuseport_detach_sock);
> > >  
> > > diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
> > > index 1451aa9712b0..b27241ea96bd 100644
> > > --- a/net/ipv4/inet_connection_sock.c
> > > +++ b/net/ipv4/inet_connection_sock.c
> > > @@ -992,6 +992,36 @@ struct sock *inet_csk_reqsk_queue_add(struct sock *sk,
> > >  }
> > >  EXPORT_SYMBOL(inet_csk_reqsk_queue_add);
> > >  
> > > +void inet_csk_reqsk_queue_migrate(struct sock *sk, struct sock *nsk)
> > > +{
> > > +	struct request_sock_queue *old_accept_queue, *new_accept_queue;
> > > +
> > > +	old_accept_queue = &inet_csk(sk)->icsk_accept_queue;
> > > +	new_accept_queue = &inet_csk(nsk)->icsk_accept_queue;
> > > +
> > > +	spin_lock(&old_accept_queue->rskq_lock);
> > > +	spin_lock(&new_accept_queue->rskq_lock);
> > I am also not very thrilled on this double spin_lock.
> > Can this be done in (or like) inet_csk_listen_stop() instead?
> 
> It will be possible to migrate sockets in inet_csk_listen_stop(), but I
> think it is better to do it just after reuseport_detach_sock() becuase we
> can select a different listener (almost) every time at a lower cost by
> selecting the moved socket and pass it to inet_csk_reqsk_queue_migrate()
> easily.
I don't see the "lower cost" point.  Please elaborate.

> 
> sk_hash of the listener is 0, so we would have to generate a random number
> in inet_csk_listen_stop().
If I read it correctly, it is also passing 0 as the sk_hash to
bpf_run_sk_reuseport() from reuseport_detach_sock().

Also, how is the sk_hash expected to be used?  I don't see
it in the test.