Message-ID: <1424542141.5565.46.camel@edumazet-glaptop2.roam.corp.google.com>
Subject: Re: 1e918876 breaks r8169 (linux-3.18+)
From: Eric Dumazet <eric.dumazet@gmail.com>
To: Florian Westphal <fw@strlen.de>
Cc: Tomas Szepe <szepe@pinerecords.com>,
        Francois Romieu <romieu@fr.zoreil.com>,
        Hayes Wang <hayeswang@realtek.com>, Eric Dumazet <edumazet@google.com>,
        Tom Herbert <therbert@google.com>,
        "David S. Miller" <davem@davemloft.net>,
        Marco Berizzi <pupilla@hotmail.com>, linux-kernel@vger.kernel.org,
        netdev <netdev@vger.kernel.org>
Date: Sat, 21 Feb 2015 10:09:01 -0800
In-Reply-To: <20150221174625.GA29681@breakpoint.cc>
References: <20150203100816.GA5807@louise.pinerecords.com>
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	 <20150221101512.GB17223@louise.pinerecords.com>
	 <20150221103104.GA26574@breakpoint.cc>
	 <1424535746.5565.42.camel@edumazet-glaptop2.roam.corp.google.com>
	 <20150221174625.GA29681@breakpoint.cc>
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On Sat, 2015-02-21 at 18:46 +0100, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > Hold on.
> > 
> > I believe there is one race in the way you access skb->xmit_more _after_
> > 
> > txd->opts1 = cpu_to_le32(status);
> > 
> > After this point, TX might have completed and TX completion already have
> > freed skb
> 
> Hmm, I _thought_ HW would not start xmit of this descriptor/skb until after
> 
> RTL_W8(TxPoll, NPQ);

Note this 'kick' does not provide tail ptr.

NIC basically looks at TX descriptors to find ones with the DescOwn bit
set. It stops when if find one TX descriptor _without_ DescOwn.

So what can happen here is the NIC was kicked earlier (prior transmit),
but found your TX descriptor, before you got a chance to read
skb->xmit_more

This is why we have these wmb() everywhere. We want to do

txd->opts1 = XXX

Only when we are really ready.


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