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 10:00:03 +0100
From:	"David Laight" <David.Laight@...LAB.COM>
To:	"David Miller" <davem@...emloft.net>, <yevgenyp@...lanox.co.il>
Cc:	<netdev@...r.kernel.org>
Subject: RE: [PATCH 4/4] net/mlx4_en: Use atomic counter to decide when queue is full

 
> > 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.

	David


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