netdev - Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

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Date:	Thu, 29 Nov 2012 11:55:43 +0000
From:	David Woodhouse <dwmw2@...radead.org>
To:	Krzysztof Mazur <krzysiek@...lesie.net>
Cc:	David Laight <David.Laight@...LAB.COM>,
	chas williams - CONTRACTOR <chas@....nrl.navy.mil>,
	davem@...emloft.net, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, nathan@...verse.com.au
Subject: Re: [PATCH v2 3/3] pppoatm: protect against freeing of vcc

On Thu, 2012-11-29 at 11:57 +0100, Krzysztof Mazur wrote:
> do we really need to wait here?
> Why don't just do something like that:
> 
> 	tasklet_disable(&card->tlet);
> 	spin_lock(&card->tx_queue_lock);
> 	for each skb in queue
> 		SKB_CB(skb)->vcc = NULL;
> 	spin_unlock(&card->tx_queue_lock);
> 	tasklet_enable(&card->tlet);
> 
> or if we really want to call vcc->pop() for such skbs:
> 
> 	tasklet_disable(&card->tlet);
> 	spin_lock(&card->tx_queue_lock);
> 	for each skb in queue {
> 		skb_get(skb);
> 		solos_pop(SKB_CB(skb)->vcc, skb);
> 		SKB_CB(skb)->vcc = NULL;
> 	}
> 	spin_unlock(&card->tx_queue_lock);
> 	tasklet_enable(&card->tlet);

Yes, we could certainly remove the packets from the tx_queue first.

However, in the card->using_dma case there might be a skb for this vcc
*currently* being DMA'd, and we'd still need to wait for that one.

I suppose we could just have a waitqueue in *every* TX skb, and under
card->tx_lock we could add ourselves to *that* waitqueue. Or just a
global waitqueue for DMA tx_done, perhaps. But waiting for our own
PKT_PCLOSE skb is just 'cleaner' in my view. It's simpler, and it's much
easier to test. Even if I had DMA-capable hardware, I'd have to get the
right timing to properly test that TX-pending-DMA case.

So dequeuing the packets would only serve to make pclose() slightly
faster, rather than simplifying it. It's hardly a fast path that we care
about, and I've also already ensured that there should only be one or
two packets queued per vcc *anyway*. So I'm mostly inclined not to
bother.

(I did fix the timeout argument to wait_for_completion_timeout())

-- 
dwmw2

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