netdev - Re: 1e918876 breaks r8169 (linux-3.18+)

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Message-ID: <1424542141.5565.46.camel@edumazet-glaptop2.roam.corp.google.com>
Date:	Sat, 21 Feb 2015 10:09:01 -0800
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Florian Westphal <fw@...len.de>
Cc:	Tomas Szepe <szepe@...erecords.com>,
	Francois Romieu <romieu@...zoreil.com>,
	Hayes Wang <hayeswang@...ltek.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Marco Berizzi <pupilla@...mail.com>,
	linux-kernel@...r.kernel.org, netdev <netdev@...r.kernel.org>
Subject: Re: 1e918876 breaks r8169 (linux-3.18+)

On Sat, 2015-02-21 at 18:46 +0100, Florian Westphal wrote:
> Eric Dumazet <eric.dumazet@...il.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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