netdev - Re: [PATCH net-next v5 07/10] forcedeth: implement ndo_get

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Date:	Wed, 16 Nov 2011 11:25:23 -0800
From:	David Decotigny <david.decotigny@...gle.com>
To:	Stephen Hemminger <shemminger@...tta.com>,
	Ben Hutchings <bhutchings@...arflare.com>
Cc:	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Ian Campbell <ian.campbell@...rix.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
	Jiri Pirko <jpirko@...hat.com>, Joe Perches <joe@...ches.com>,
	Szymon Janc <szymon@...c.net.pl>,
	Richard Jones <rick.jones2@...com>,
	Ayaz Abdulla <AAbdulla@...dia.com>
Subject: Re: [PATCH net-next v5 07/10] forcedeth: implement ndo_get_stats64() API

Thanks for your feedback on these concerns.

On Wed, Nov 16, 2011 at 9:14 AM, Ben Hutchings
<bhutchings@...arflare.com> wrote:
> On Wed, 2011-11-16 at 08:55 -0800, Stephen Hemminger wrote:
>> Or realize that the Tx side is single threaded by dev->xmit_lock already
>> and the Rx side is single threaded by NAPI.
>
> Right.

Yes, that's what I meant by "Each software stat field is updated by
one single writer." in a previous email. However, I think that TX and
RX paths are not always synchronized. So I'm afraid that if I'm using
a single seqcount, I might run into trouble in the absence of a lock
around each update.... and I'd really prefer to avoid such a lock. Are
you suggesting I should use 2 independent seqcounts? One for RX path,
the other for TX path, all this without a lock around writers?

Side-note: what are the bad implications of using atomic_t (in the fast paths)?
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