linux-kernel - Re: [PATCH v5 net-next 3/3] net: mscc: ocelot: use bulk reads for stats

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Date:   Tue, 8 Feb 2022 15:34:49 +0000
From:   Vladimir Oltean <vladimir.oltean@....com>
To:     Colin Foster <colin.foster@...advantage.com>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        "UNGLinuxDriver@...rochip.com" <UNGLinuxDriver@...rochip.com>,
        Alexandre Belloni <alexandre.belloni@...tlin.com>,
        Claudiu Manoil <claudiu.manoil@....com>
Subject: Re: [PATCH v5 net-next 3/3] net: mscc: ocelot: use bulk reads for
 stats

On Tue, Feb 08, 2022 at 05:03:03PM +0200, Vladimir Oltean wrote:
> >  	for (i = 0; i < ocelot->num_phys_ports; i++) {
> > +		unsigned int idx = 0;
> > +
> 
> This is a bug which causes ocelot->stats to be overwritten with the
> statistics of port 0, for all ports. Either move the variable
> declaration and initialization with 0 in the larger scope (outside the
> "for" loop), or initialize idx with i * ocelot->num_stats.

My analysis was slightly incorrect. Somehow I managed to fool myself
into thinking that you had tested this in a limited scenario, hence the
reason you didn't notice it's not working. But apparently you didn't
test with traffic at all.

So ocelot->stats isn't overwritten with the stats of port 0 for all
ports. But rather, all ports write into the ocelot->stats space
dedicated for port 0, effectively overwriting the stats of port 0 with
the stats of the last port. And no one populates the ocelot->stats space
for ports [1 .. last]. So no port has good statistics, I don't see a
circumstance where testing could have misled you.