netdev - Re: Warning in net/ipv4/af

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Message-ID: <1274851115.25136.7.camel@edumazet-laptop>
Date:	Wed, 26 May 2010 07:18:35 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Anton Blanchard <anton@...ba.org>
Cc:	netdev@...r.kernel.org
Subject: Re: Warning in net/ipv4/af_inet.c:154

Le mercredi 26 mai 2010 à 13:19 +1000, Anton Blanchard a écrit :
> Hi,
> 
> > > Which is:
> > > 
> > >         WARN_ON(sk->sk_forward_alloc);
> > > 
> > 
> > Yes, the infamous one :)
> > 
> > Is it reproductible ? What kind of workload is it ?
> > What is the NIC involved ?
> 
> It was running sysbench against a postgresql database over localhost. In
> each case I checked, sk_forward_alloc was less than one page.

ok. I am a bit surprised postgresql uses UDP

> 
> I notice we update sk_forward_alloc in sk_mem_charge and sk_mem_uncharge.
> Since it isn't an atomic variable I went looking for a lock somewhere in
> the call chain (first thought was the socket lock). I couldn't find
> anything, but I could easily be missing something.
> 

UDP path uses socket lock indeed to protect sk_forward_alloc non atomic
updates.

Check skb_free_datagram_locked() for example : sk_mem_reclaim_partial()
is called inside the lock_sock_bh/unlock_sock_bh protected section, and
*after* the uncharge done by skb_orphan(). This function was changed
recently, so maybe your platform hit some bug somewhere.

In receive path, we hold the socket lock while calling
sock_queue_rcv_skb()



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