netdev - RE: [PATCH 4/4] net/mlx4_en: Use atomic counter to decide when queue is full

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Date:	Mon, 25 Jun 2012 12:12:05 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	David Miller <davem@...emloft.net>, yevgenyp@...lanox.co.il,
	netdev@...r.kernel.org
Subject: RE: [PATCH 4/4] net/mlx4_en: Use atomic counter to decide when
 queue is full

On Mon, 2012-06-25 at 10:00 +0100, David Laight wrote:
> > > The Transmit and transmit completion flows execute from different
> contexts,
> > > which are not synchronized. Hence naive reading the of consumer
> index might
> > > give wrong value by the time it is being used, That could lead to a
> state of transmit timeout.
> > > Fix that by using atomic variable to maintain that index.
> > > 
> > > Signed-off-by: Yevgeny Petrilin <yevgenyp@...lanox.co.il>
> > 
> > I'm not convinced.  There is only one place that actually changes
> > the counter.
> > 
> > So it seems more like you have a missing memory barrier somewhere.
> 
> Or just keep the two ring indexes - instead of keeping the
> number of 'active' entries as well.
> Then you don't have a variable which the tx setup and
> tx completion routines both update.

This is what was implied by David.

Using a producer/consumer index and appropriate memory barriers.

start_xmit() and tx completion can be truly lockless and atomicless in
their fast path.

There are many drivers doing that correctly.

tg3 driver is a good example.



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