netdev - Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

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Message-ID: <CANn89iKKJrUUmCzAue1N375TLCHZunnyNwkpJdxbk=RdeeZQwg@mail.gmail.com>
Date:   Mon, 25 Apr 2022 10:24:40 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Lukas Wunner <lukas@...ner.de>
Cc:     Jann Horn <jannh@...gle.com>, Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        Oliver Neukum <oneukum@...e.com>,
        "David S. Miller" <davem@...emloft.net>,
        Oleksij Rempel <o.rempel@...gutronix.de>,
        netdev <netdev@...r.kernel.org>,
        USB list <linux-usb@...r.kernel.org>,
        Andrew Lunn <andrew@...n.ch>,
        Jacky Chou <jackychou@...x.com.tw>, Willy Tarreau <w@....eu>,
        Lino Sanfilippo <LinoSanfilippo@....de>,
        Philipp Rosenberger <p.rosenberger@...bus.com>,
        Heiner Kallweit <hkallweit1@...il.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH] net: linkwatch: ignore events for unregistered netdevs

On Mon, Apr 25, 2022 at 10:20 AM Lukas Wunner <lukas@...ner.de> wrote:
>
> On Mon, Apr 25, 2022 at 05:18:51PM +0200, Jann Horn wrote:
> > Well, it's not quite a refcount. It's a count that can be incremented
> > and decremented but can't be read while the device is alive, and then
> > at some point it turns into a count that can be read and decremented
> > but can't be incremented
>
> Pardon me for being dense, but most other subsystems use the refcounting
> built into struct device (or rather, its kobject) and tear it down
> when the refcount reaches zero (e.g. pci_release_dev(), spidev_release()).
>
> What's the rationale for struct net_device rolling its own refcounting?
> Historic artifact?


Yes, probably. This was there way before new fancy mechanisms were invented.

>
>
> I think a lot of these issues would solve themselves if that was done away
> with and replaced with the generic kobject refcounting.  It's a pity that
> the tracking infrastructure is now netdev-specific and other subsystems
> cannot benefit from it.

Make sure that whatever replaces it, heavy dev_hold()/dev_put() users
do not come to a crawl.

af_packet is using this stuff.

Some users want to send millions of packets per second, without having
to bypass the kernel because it is suddenly too slow.


>
> Thanks,
>
> Lukas