Re: [PATCH net] tcp: avoid creating multiple req socks with the same tuples

From: Zhiqiang Liu
Date: Wed Jun 05 2019 - 04:56:49 EST

å 2019/6/4 23:24, Eric Dumazet åé:
> On Tue, Jun 4, 2019 at 7:47 AM Mao Wenan <maowenan@xxxxxxxxxx> wrote:
>> There is one issue about bonding mode BOND_MODE_BROADCAST, and
>> two slaves with diffierent affinity, so packets will be handled
>> by different cpu. These are two pre-conditions in this case.

>> Signed-off-by: Mao Wenan <maowenan@xxxxxxxxxx>
>> --
> This issue has been discussed last year.
> I am afraid your patch does not solve all races.
> The lookup you add is lockless, so this is racy.
> Really the only way to solve this is to make sure that _when_ the
> bucket lock is held,
> we do not insert a request socket if the 4-tuple is already in the
> chain (probably in inet_ehash_insert())
> This needs more tricky changes than your patch.

This kind case is rarely used, and the condition of the issue is strict.
If we add the "lookup" before or in inet_ehash_insert func for each reqsk,
overall performance will be affected.

We may solve the small probability issue with a trick in the tcp_v4_rcv.
If the ACK is invalid checked by tcp_check_req func, the req could be dropped,
and then goto the lookup for searching another avaliable reqsk. In this way,
the performance will not be affected in the normal process.

The patch is given as following:
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index a2896944aa37..9d0491587ed2 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1874,8 +1874,10 @@ int tcp_v4_rcv(struct sk_buff *skb)
goto discard_and_relse;
if (nsk == sk) {
- reqsk_put(req);
+ inet_csk_reqsk_queue_drop_and_put(sk, req);
+ sock_put(sk);
+ goto lookup;
} else if (tcp_child_process(sk, nsk, skb)) {
tcp_v4_send_reset(nsk, skb);
goto discard_and_relse;