netdev - RE: Strange latency spikes/TX network stalls on Sun Fire X4150(x86) and e1000e

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Message-ID: <1338375895.2760.153.camel@edumazet-glaptop>
Date:	Wed, 30 May 2012 13:04:55 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	David Laight <David.Laight@...LAB.COM>
Cc:	Hiroaki SHIMODA <shimoda.hiroaki@...il.com>,
	Tom Herbert <therbert@...gle.com>,
	Denys Fedoryshchenko <denys@...p.net.lb>,
	netdev@...r.kernel.org, e1000-devel@...ts.sourceforge.net,
	jeffrey.t.kirsher@...el.com, jesse.brandeburg@...el.com,
	davem@...emloft.net
Subject: RE: Strange latency spikes/TX network stalls on Sun Fire
 X4150(x86) and e1000e

On Wed, 2012-05-30 at 11:52 +0100, David Laight wrote:
> > > +	num_queued = dql->num_queued;
> > 
> > 
> > I suggest :
> > 
> > 	num_queued = ACCESS_ONCE(dql->num_queued);
> > 	
> > Or else compiler is free to do whatever he wants.
> 
> Or make the structure member volatile, then the
> compiler can only read it once.

No. Compiler can read it several times. Really.

> Probably worth while if the value is expected to
> be read like that.

Please don't use lazy volatile.

Unless you really want Linus flames (and ours)

ACCESS_ONCE() is much cleaner.


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