[Devel] Re: [patch 3/3][NETNS45][V2] remove timewait sockets at cleanup

[Daniel Lezcano]

Denis V. Lunev wrote:
> Daniel Lezcano wrote:
>> From: Daniel Lezcano <dlezcano at fr.ibm.com>
>>
>> Denis Lunev spotted that if we take a reference to the network namespace
>> with the timewait sockets, we will need to wait for their expiration to
>> have the network namespace freed. This is a waste of time, the timewait
>> sockets are for avoiding to receive a duplicate packet from the network,
>> if the network namespace is freed, the network stack is removed, so no
>> chance to receive any packets from the outside world.
>>
>> This patchset remove/destroy the timewait sockets when the
>> network namespace is freed.
>>
>> Signed-off-by: Daniel Lezcano <dlezcano at fr.ibm.com>
>> ---
>>  net/ipv4/tcp.c |   53 +++++++++++++++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 53 insertions(+)
>>
> 
> [...]
> This place seems non-trival and broken for me :( May be I am wrong.
>> + 		write_lock_bh(&head->lock);
>> +
>> +		sk_for_each_safe(sk, node, tmp, &head->twchain) {
>> +
>> +			tw = inet_twsk(sk);
>> +			if (tw->tw_net != net)
>> +				continue;
>> +
>> +			/* deschedule the timewait socket */
>> +			spin_lock(&tcp_death_row.death_lock);
>> +			if (inet_twsk_del_dead_node(tw)) {
>> +				inet_twsk_put(tw);
>> +				if (--tcp_death_row.tw_count == 0)
>> +					del_timer(&tcp_death_row.tw_timer);
> There is a call inet_twsk_deschedule which do exactly what we need to
> 
> void inet_twsk_deschedule(struct inet_timewait_sock *tw,
>                           struct inet_timewait_death_row *twdr)
> {
>         spin_lock(&twdr->death_lock);
>         if (inet_twsk_del_dead_node(tw)) {
>                 inet_twsk_put(tw);
>                 if (--twdr->tw_count == 0)
>                         del_timer(&twdr->tw_timer);
>         }
>         spin_unlock(&twdr->death_lock);
>         __inet_twsk_kill(tw, twdr->hashinfo);
> }
> 
> and, from my point of view, your patch [2] is even not needed. We should do
> 
> restart:
>         write_lock_bh(&head->lock);
>         sk_for_each_safe(sk, node, tmp, &head->twchain) {
>             tw = inet_twsk(sk);
>             if (tw->tw_net != net)
>                 continue;
>             sock_hold(sk);
>             write_unlock_bh(&head->lock);
>             inet_twsk_deschedule(tw, &tcp_death_row);
>             inet_twsk_put(tw);
>             goto restart;
>         }
> 
> This removes serious locking issue. You have introduced dependency
> between write_lock_bh(&head->lock); and
> spin_lock(&tcp_death_row.death_lock);
> This should be at least checked and documented in the headers. I am not
> sure that this is correct.
> If my approach is correct, your second patch is not needed.
> 
> It will also worth to local_bh_enable() at the very beginning and remove
> _bh from write_lock.

local_bh_disable / local_bh_enable at the very beginning ?

like that ?

-------------

local_bh_disable();

/* Browse the the established hash table */
for (h = 0; h < (tcp_hashinfo.ehash_size); h++) {
                struct inet_ehash_bucket *head =
                        inet_ehash_bucket(&tcp_hashinfo, h);
restart:
          write_lock(&head->lock);
          sk_for_each_safe(sk, node, tmp, &head->twchain) {
              tw = inet_twsk(sk);
              if (tw->tw_net != net)
                  continue;
              sock_hold(sk);
              write_unlock(&head->lock);
              inet_twsk_deschedule(tw, &tcp_death_row);
              inet_twsk_put(tw);
              goto restart;
          }	
}

local_bh_enable();

-------------