netdev - Re: [PATCH net v2] tcp: Don't acquire inet_listen

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Date:   Thu, 9 Dec 2021 12:06:32 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
CC:     Jakub Kicinski <kuba@...nel.org>,
        Kuniyuki Iwashima <kuniyu@...zon.co.jp>,
        <eric.dumazet@...il.com>, <davem@...emloft.net>,
        <dsahern@...nel.org>, <efault@....de>, <netdev@...r.kernel.org>,
        <tglx@...utronix.de>, <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH net v2] tcp: Don't acquire inet_listen_hashbucket::lock
 with disabled BH.

On Mon, Dec 06, 2021 at 01:02:16PM +0100, Sebastian Andrzej Siewior wrote:
> Commit
>    9652dc2eb9e40 ("tcp: relax listening_hash operations")
> 
> removed the need to disable bottom half while acquiring
> listening_hash.lock. There are still two callers left which disable
> bottom half before the lock is acquired.
> 
> On PREEMPT_RT the softirqs are preemptible and local_bh_disable() acts
> as a lock to ensure that resources, that are protected by disabling
> bottom halves, remain protected.
> This leads to a circular locking dependency if the lock acquired with
> disabled bottom halves is also acquired with enabled bottom halves
> followed by disabling bottom halves. This is the reverse locking order.
> It has been observed with inet_listen_hashbucket::lock:
> 
> local_bh_disable() + spin_lock(&ilb->lock):
>   inet_listen()
>     inet_csk_listen_start()
>       sk->sk_prot->hash() := inet_hash()
> 	local_bh_disable()
> 	__inet_hash()
> 	  spin_lock(&ilb->lock);
> 	    acquire(&ilb->lock);
> 
> Reverse order: spin_lock(&ilb->lock) + local_bh_disable():
>   tcp_seq_next()
>     listening_get_next()
>       spin_lock(&ilb->lock);
The net tree has already been using ilb2 instead of ilb.
It does not change the problem though but updating
the commit log will be useful to avoid future confusion.

iiuc, established_get_next() does not hit this because
it calls spin_lock_bh() which then keeps the
local_bh_disable() => spin_lock() ordering?

The patch lgtm.
Acked-by: Martin KaFai Lau <kafai@...com>

> 	acquire(&ilb->lock);
> 
>   tcp4_seq_show()
>     get_tcp4_sock()
>       sock_i_ino()
> 	read_lock_bh(&sk->sk_callback_lock);
> 	  acquire(softirq_ctrl)	// <---- whoops
> 	  acquire(&sk->sk_callback_lock)
> 
> Drop local_bh_disable() around __inet_hash() which acquires
> listening_hash->lock. Split inet_unhash() and acquire the
> listen_hashbucket lock without disabling bottom halves; the inet_ehash
> lock with disabled bottom halves.