netdev - Re: [RFC] tulip: Support for byte queue limits

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Date:	Wed, 17 Jul 2013 16:42:07 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	George Spelvin <linux@...izon.com>
CC:	<eric.dumazet@...il.com>, <grantgrundler@...il.com>,
	<grundler@...isc-linux.org>, <netdev@...r.kernel.org>
Subject: Re: [RFC] tulip: Support for byte queue limits

On Wed, 2013-07-17 at 00:09 -0400, George Spelvin wrote:
> >>  	wmb();
> >>  
> >> -	tp->cur_tx++;
> >> -
> >>  	/* Trigger an immediate transmit demand. */
> >>  	iowrite32(0, tp->base_addr + CSR1);
> >>  
> >> +	tp->cur_tx++;
> >> +	netdev_sent_queue(dev, skb->len);
> >>  	spin_unlock_irqrestore(&tp->lock, flags);
> 
> > This is not good practice, because once you start DMA you have
> > effectively passed ownership of the skb to the TX completion handler.
> 
> Thank you for this advice.  Just to be clear, is the only issue reading
> skb->len from a potentially deallocated skb?

That's what I was thinking of.

> Or is it also going go
> give the byte queue system fits if the transmit complete handler calls
> netdev_completed_queue before the transmitter calls netdev_sent_queue?

I don't know.

> I'd hope it can underflow safely, and only look at the net value after
> the transmit handler returns.
> 
> > Presumably the TX completion handler will hold this spinlock and
> > therefore cannot free the skb before you use skb->len above.  So this
> > will be safe now.  But one day someone may want to get rid of this lock,
> > so this is a trap waiting to spring.
> 
> Sounds like a fun project.  But I have to dig into the BQL code; if it's
> getting and dropping locks inside netdev_*_queue, the win is limited.
> 
> (A lock-free version would have separate "sent" and "completed" counters,
> and compute the difference when a snapshot is required.)

Yes, it is lock-free with separate counters.  The implementation is in
lib/dynamic_queue_limits.c.  I think that the use of POSDIFF() protects
against races that could leave completed > sent.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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