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Date:	Thu, 24 May 2012 16:51:17 +0200
From:	Eric Dumazet <>
	Jesper Dangaard Brouer <>
Cc:	David Miller <>, Martin Topholm <>,
	netdev <>,
	Tom Herbert <>
Subject: Re: [RFC PATCH] tcp: Fast/early SYN handling to mitigate SYN floods

On Thu, 2012-05-24 at 15:26 +0200, Christoph Paasch wrote:
> Hello,
> On 05/24/2012 03:01 PM, Jesper Dangaard Brouer wrote:
> > I have been doing some TCP performance measurements with SYN flooding,
> > and have found that, we don't handle this case well.
> > 
> > I have made a patch for fast/early SYN handling in tcp_v4_rcv() in
> > net/ipv4/tcp_ipv4.c.  This increases SYN performance from 130 kpps to
> > 750 kpps (max of the generator), with idle CPU cycles.
> > 
> > Current locking:
> >  During a SYN flood (against a single port) all CPUs are spinning on
> > the same spinlock, namely bh_lock_sock_nested(sk), in tcp_ipv4.c.  The
> > lock dates back to a commit by DaveM in May 1999, see historic
> > commit[1].  It seem that TCP runs fully locked, per sock.
> > 
> > I need some help with locking, as the patch seems to work fine, with
> > NO-PREEMPT, but with PREEMPT enabled I start to see warnings (in
> > reqsk_queue_destroy) and oopses (in inet_csk_reqsk_queue_prune).
> > 
> > What am I missing?
> For each retransmission of a SYN you will add a request-sock to the
> syn_table, because you do not pass by tcp_v4_hnd_req(), which checks
> this by calling inet_csk_search_req().
> And your warning in reqsk_queue_destroy is because the access to the the
> request_sock_queue is no more protected by a lock.
> The request_sock_queue is a shared resource, which must be protect by a
> lock. As you allow "parallel" SYN-processing, the queue will get corrupted.

Hi guys, that's a very interesting subject.

I began work on fully converting this stuff to RCU some weeks ago but
got distracted by codel / fq_codel and other cool stuff (TCP coalescing
and skb->frag_head)

I dont know if you remember the SO_REUSEPORT patch(s) posted by Tom
Herbert in the past. The remaining issue was about adding/removing a new
listener to a pool of listeners to same port, and hash function was
changed so we could lost some connexions in SYN_RECV state at this

So I was working having a shared table, and not anymore using a central
spinlock, but an array of spinlock, as done elsewhere

My work is probably a ~500 LOC target, allowing concurrent processing by
all cpus of the host.

Jesper, my goals are probably different than yours, unless I
misunderstood your intention.

I feel you want to have an emergency mode, when listener is overflowed
to immediately send a SYNCOOKIE ?

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