Re: [PATCH net v3] net: stmmac: protect updates of 64-bit statistics counters

From: "Petr Tesařík" <petr@tesarici.cz>
To: "Linux regression tracking (Thorsten Leemhuis)"
	<regressions@leemhuis.info>, Eric Dumazet <edumazet@google.com>
Cc: Linux regressions mailing list <regressions@lists.linux.dev>,
	"David S. Miller" <davem@davemloft.net>,
	Paolo Abeni <pabeni@redhat.com>, Jakub Kicinski <kuba@kernel.org>,
	Jisheng Zhang <jszhang@kernel.org>,
	Alexandre Torgue <alexandre.torgue@foss.st.com>,
	Jose Abreu <joabreu@synopsys.com>,
	Maxime Coquelin <mcoquelin.stm32@gmail.com>,
	Chen-Yu Tsai <wens@csie.org>,
	Jernej Skrabec <jernej.skrabec@gmail.com>,
	Samuel Holland <samuel@sholland.org>,
	"open list:STMMAC ETHERNET DRIVER" <netdev@vger.kernel.org>,
	"moderated list:ARM/STM32 ARCHITECTURE"
	<linux-stm32@st-md-mailman.stormreply.com>,
	"moderated list:ARM/STM32 ARCHITECTURE"
	<linux-arm-kernel@lists.infradead.org>,
	open list <linux-kernel@vger.kernel.org>,
	"open list:ARM/Allwinner sunXi SoC support"
	<linux-sunxi@lists.linux.dev>,
	Marc Haber <mh+netdev@zugschlus.de>, Andrew Lunn <andrew@lunn.ch>,
	Florian Fainelli <f.fainelli@gmail.com>,
	stable@vger.kernel.org, alexis.lothore@bootlin.com,
	Guenter Roeck <linux@roeck-us.net>
Subject: Re: [PATCH net v3] net: stmmac: protect updates of 64-bit statistics counters
Date: Wed, 6 Mar 2024 11:03:12 +0100	[thread overview]
Message-ID: <20240306110312.04ddcde3@meshulam.tesarici.cz> (raw)
In-Reply-To: <20240306100153.32d305f7@meshulam.tesarici.cz>

On Wed, 6 Mar 2024 10:01:53 +0100
Petr Tesařík <petr@tesarici.cz> wrote:

> On Wed, 6 Mar 2024 09:23:53 +0100
> "Linux regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info> wrote:
> 
> > On 28.02.24 12:03, Petr Tesařík wrote:  
> > > On Wed, 28 Feb 2024 07:19:56 +0100
> > > "Linux regression tracking (Thorsten Leemhuis)" <regressions@leemhuis.info> wrote:
> > >     
> > >> Net maintainers, chiming in here, as it seems handling this regression
> > >> stalled.    
> > > Indeed, I was too busy with sandbox mode...    
> > 
> > Hmm, no reply in the past week to Petr's request for help from someone
> > with more knowledge about the field. :-/
> > 
> > So I guess this means that this won't be fixed for 6.8? Unfortunate, but
> > well, that's how it it sometimes.  
> 
> For the record, I _can_ reproduce lockdep splats on my device, but they
> don't make any sense to me. They seem to confirm Jisheng Zhang's
> conclusion that lockdep conflates two locks which should have different
> lock-classes.
> 
> So far I have noticed only one issue: the per-cpu syncp's are not
> initialized. I'll recompile and see if that's what confuses lockdep.

That wasn't the issue. FTR the syncp was in fact initialized, because
devm_netdev_alloc_pcpu_stats() is a macro that also takes care of the
initialization of the syncp struct field.

The problem is u64_stats_init().

Commit 9464ca650008 ("net: make u64_stats_init() a function") changed
it to an inline function. But that's wrong. It uses seqcount_init(),
which in turn declares:

	static struct lock_class_key __key;

This assumes that each lock gets its own instance. But if
u64_stats_init() is a function (albeit an inline one), all calls
within the same file end up using the same instance.

Eric, would it be OK to revert the above-mentioned commit?

Petr T