netdev - Re: [PATCH 2/2] mv643xx_eth: fix race in trasmit path.

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Message-ID: <1308587517.29160.7.camel@edumazet-HP-Compaq-6005-Pro-SFF-PC>
Date:	Mon, 20 Jun 2011 18:31:57 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	mbizon@...ebox.fr
Cc:	Richard Cochran <richardcochran@...il.com>, netdev@...r.kernel.org,
	David Miller <davem@...emloft.net>, stable@...nel.org,
	Lennert Buytenhek <buytenh@...tstofly.org>
Subject: Re: [PATCH 2/2] mv643xx_eth: fix race in trasmit path.

Le lundi 20 juin 2011 à 16:07 +0200, Maxime Bizon a écrit :
> On Mon, 2011-06-20 at 09:48 +0200, Richard Cochran wrote:
> 
> Hi Richard,
> 
> > Because the socket buffer is freed in the completion interrupt, it is not
> > safe to access it after submitting it to the hardware.
> 
> I don't see why.
> 
> skb is freed from txq_reclaim() which grabs the tx queue lock before,
> (hence the lockless __skb_queue_xxx() in both functions)
> 
> What am I missing ?

You are right, this driver still takes tx queue lock in its TX
completion path.

txq_reclaim() can currently run for a very long time, blocking other
cpus to make progress if blocked on tx queue lock.

So consider this patch as a cleanup :

Its better to make all drivers behave the same way

1) Either increment tx stats in their start_xmit(), and making sure they
dont access skb->len too late.

2) increment tx stats in their TX completion path.

Then we can work on making TX completion path not taking tx queue lock
in its fast path.


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