netdev - Re: Aw: Re: [PATCH] mlx4: give precise rx/tx bytes/packets counters

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Date:   Fri, 25 Nov 2016 11:19:11 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Lino Sanfilippo <LinoSanfilippo@....de>
Cc:     David Laight <David.Laight@...LAB.COM>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: Aw: Re: [PATCH] mlx4: give precise rx/tx bytes/packets counters

On Fri, 2016-11-25 at 17:30 +0100, Lino Sanfilippo wrote:
> Hi,
> 
> 
> > 
> > The READ_ONCE() are documenting the fact that no lock is taken to fetch
> > the stats, while another cpus might being changing them.
> > 
> > I had no answer yet from https://patchwork.ozlabs.org/patch/698449/
> > 
> > So I thought it was not needed to explain this in the changelog, given
> > that it apparently is one of the few things that can block someone to
> > understand one of my changes :/
> > 
> > Apparently nobody really understands READ_ONCE() purpose, it is really a
> > pity we have to explain this over and over.
> > 
> 
> Even at the risk of showing once more a lack of understanding for
> READ_ONCE():
> Does not a READ_ONCE() have to e paired with some kind of
> WRITE_ONCE()? 

You are right.

Although in this case, the producers are using a lock, and do

ring->packets++;

We hopefully have compilers/cpus that do not put intermediate garbage in
ring->packets while doing the increment.

One problem with :

WRITE_ONCE(ring->packets, ring->packets + 1);

is that gcc no longer uses an INC instruction.

Maybe we need some ADD_ONCE(ptr, val) macro doing the proper thing.

> Furthermore: there a quite some network drivers that ensure visibility
> of 
> the descriptor queue indices between xmit and xmit completion function
> by means of
> smp barriers. Could all these drivers theoretically be adjusted to use
> READ_ONCE(),
> WRITE_ONCE() for the indices instead?
> 

Well, for this precise case we do need appropriate smp barriers.

READ_ONCE() can be better than poor barrier(), look at 
https://git.kernel.org/cgit/linux/kernel/git/davem/net-next.git/commit/?id=b668534c1d9b80f4cda4d761eb11d3a6c9f4ced8