netdev - Re: [PATCH net-next] tcp: try to keep packet if SYN

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Date:   Wed, 14 Feb 2018 14:22:33 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     eric.dumazet@...il.com
Cc:     netdev@...r.kernel.org, soukjin.bae@...sung.com
Subject: Re: [PATCH net-next] tcp: try to keep packet if SYN_RCV race is
 lost

From: Eric Dumazet <eric.dumazet@...il.com>
Date: Tue, 13 Feb 2018 06:14:12 -0800

> From: Eric Dumazet <edumazet@...gle.com>
> 
> 배석진 reported that in some situations, packets for a given 5-tuple
> end up being processed by different CPUS.
> 
> This involves RPS, and fragmentation.
> 
> 배석진 is seeing packet drops when a SYN_RECV request socket is
> moved into ESTABLISH state. Other states are protected by socket lock.
> 
> This is caused by a CPU losing the race, and simply not caring enough.
> 
> Since this seems to occur frequently, we can do better and perform
> a second lookup.
> 
> Note that all needed memory barriers are already in the existing code,
> thanks to the spin_lock()/spin_unlock() pair in inet_ehash_insert()
> and reqsk_put(). The second lookup must find the new socket,
> unless it has already been accepted and closed by another cpu.
> 
> Note that the fragmentation could be avoided in the first place by
> use of a correct TCP MSS option in the SYN{ACK} packet, but this
> does not mean we can not be more robust.
> 
> Many thanks to 배석진 for a very detailed analysis.
> 
> Reported-by: 배석진 <soukjin.bae@...sung.com>
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>

Applied, thanks Eric.