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

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Date:	Wed, 16 Nov 2011 21:32:33 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.com>
Cc:	David Decotigny <david.decotigny@...gle.com>,
	Stephen Hemminger <shemminger@...tta.com>,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	"David S. Miller" <davem@...emloft.net>,
	Ian Campbell <ian.campbell@...rix.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

Le mercredi 16 novembre 2011 à 19:32 +0000, Ben Hutchings a écrit :
> On Wed, 2011-11-16 at 11:25 -0800, David Decotigny wrote:
> > 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?
> 
> Yes, there should be 1 for each path.

David, drivers/net/ethernet/marvell/sky2.c contains code like that
(different syncpfor rx/tx)

TX path:
                        u64_stats_update_begin(&sky2->tx_stats.syncp);
                        ++sky2->tx_stats.packets;
                        sky2->tx_stats.bytes += skb->len;
                        u64_stats_update_end(&sky2->tx_stats.syncp);


RX path:

        u64_stats_update_begin(&sky2->rx_stats.syncp);
        sky2->rx_stats.packets += packets;
        sky2->rx_stats.bytes += bytes;
        u64_stats_update_end(&sky2->rx_stats.syncp);


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