netdev - Re: [REGRESSION] [PATCH net-next v5 2/2] net: stmmac: use per-queue 64 bit statistics where necessary

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Message-ID: <20230913201212.eiedub5rsztuwaa7@pengutronix.de>
Date: Wed, 13 Sep 2023 22:12:12 +0200
From: Uwe Kleine-König <u.kleine-koenig@...gutronix.de>
To: Jisheng Zhang <jszhang@...nel.org>
Cc: Johannes Berg <johannes@...solutions.net>,
	Paolo Abeni <pabeni@...hat.com>, kernel@...gutronix.de,
	Samuel Holland <samuel@...lland.org>, netdev@...r.kernel.org,
	linux-stm32@...md-mailman.stormreply.com,
	Alexandre Torgue <alexandre.torgue@...s.st.com>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	linux-kernel@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>,
	Jose Abreu <joabreu@...opsys.com>,
	Maxime Coquelin <mcoquelin.stm32@...il.com>,
	Giuseppe Cavallaro <peppe.cavallaro@...com>,
	Chen-Yu Tsai <wens@...e.org>, linux-sunxi@...ts.linux.dev,
	"David S . Miller" <davem@...emloft.net>,
	linux-arm-kernel@...ts.infradead.org,
	Lucas Stach <l.stach@...gutronix.de>
Subject: Re: [REGRESSION] [PATCH net-next v5 2/2] net: stmmac: use per-queue
 64 bit statistics where necessary

Hello,

On Wed, Sep 13, 2023 at 10:34:42PM +0800, Jisheng Zhang wrote:
> On Tue, Sep 12, 2023 at 11:30:14AM +0200, Johannes Berg wrote:
> > On Tue, 2023-09-12 at 11:24 +0200, Uwe Kleine-König wrote:
> > > > 
> > > > The newly added "struct u64_stats_sync syncp" uses a seqlock
> > > > internally, which is broken into multiple words on 32bit machines, and
> > > > needs to be initialized properly. You need to call u64_stats_init on
> > > > syncp before first usage.
> > > 
> > > This is done. The problematic thing is that in stmmac_open() ->
> > > __stmmac_open() the syncp initialized before is overwritten by
> > > 
> > > 	memcpy(&priv->dma_conf, dma_conf, sizeof(*dma_conf));
> 
> Thank Johannes and Uwe for pointing out the issue.
> 
> > > 
> > > Do I need to point out that this is ugly?
> > 
> > I think it also leaks the (lockdep) state since it reinits the syncp
> > (and a lot of other state) doing this. This is also called when the MTU
> > changes.
> > 
> > Also, I couldn't convince myself that it's even race-free? Even if it
> > is, it's not really obvious, IMHO.
> > 
> > So it seems to me that really this needs to be split into data that
> > actually should be reinitialized, and data that shouldn't, or just not
> > use memcpy() here but copy only the relevant state?
> 
> Since we are in rc1, I need to fix the bug with as small changes as
> possible. so another solution could be: replace rx/tx stats structure
> with pointers, then setup pointers in the new allocated dma_conf with
> the old one as current code did for dma_tx_size/dma_rx_size in
> stmmac_setup_dma_desc():
> 
> dma_conf->dma_tx_size = priv->dma_conf.dma_tx_size
> 
> Is it acceptable?

I wondered if you can just initialize the data directly in *priv, instead
of setting up a local copy, initialize that one + copy it over?!

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | https://www.pengutronix.de/ |

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