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Date:   Thu, 3 Dec 2020 08:35:17 -0800
From:   Jakub Kicinski <kuba@kernel.org>
To:     Oleksij Rempel <o.rempel@pengutronix.de>
Cc:     Andrew Lunn <andrew@lunn.ch>,
        Vivien Didelot <vivien.didelot@gmail.com>,
        Florian Fainelli <f.fainelli@gmail.com>,
        Vladimir Oltean <olteanv@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Russell King <linux@armlinux.org.uk>,
        Pengutronix Kernel Team <kernel@pengutronix.de>,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-mips@vger.kernel.org
Subject: Re: [PATCH v3 net-next 2/2] net: dsa: qca: ar9331: export stats64
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On Thu, 3 Dec 2020 09:50:11 +0100 Oleksij Rempel wrote:
> @Jakub,
> 
> > You can't take sleeping locks from .ndo_get_stats64.
> > 
> > Also regmap may sleep?
> > 
> > +	ret = regmap_read(priv->regmap, reg, &val);  
> 
> Yes. And underling layer is mdio bus which is by default sleeping as
> well.
> 
> > Am I missing something?  
> 
> In this log, the  ar9331_get_stats64() was never called from atomic or
> irq context. Why it should not be sleeping?

You missed some long discussions about this within last week on netdev.
Also Documentation/networking/statistics.rst.

To answer your direct question - try:

# cat /proc/net/dev

procfs iterates over devices while holding only an RCU read lock.