netdev - Re: Long stalls creating a new netns after a netns with a SMB client exits

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Date:   Mon, 31 Jul 2017 17:55:52 -0600
From:   David Ahern <dsahern@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Rolf Neugebauer <rolf.neugebauer@...ker.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Long stalls creating a new netns after a netns with a SMB client
 exits

On 7/31/17 4:01 PM, Cong Wang wrote:
>>> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
>>> index 3a19ea28339f..37db087b6c97 100644
>>> --- a/net/ipv4/tcp_ipv4.c
>>> +++ b/net/ipv4/tcp_ipv4.c
>>> @@ -1855,7 +1855,7 @@ void inet_sk_rx_dst_set(struct sock *sk, const
>>> struct sk_buff *skb)
>>>  {
>>>         struct dst_entry *dst = skb_dst(skb);
>>>
>>> -       if (dst && dst_hold_safe(dst)) {
>>> +       if (0 && dst && dst_hold_safe(dst)) {
>>>                 sk->sk_rx_dst = dst;
>>>                 inet_sk(sk)->rx_dst_ifindex = skb->skb_iif;
>>>         }
>>
>>
>> This removes the 200s stall (the test is IPv4/TCP based)
> 
> 
> Interesting. This means we have a kernel socket which holds
> the dst refcnt.

Right now there is no tracking that I am aware of for a dst cached on
the socket (outside of walking all sockets). I have been bitten by it
several times in trying to make various changes. It's basically a hidden
reference for the device.