netdev - Re: What is the purpose of dev->gflags?

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Message-ID: <20220411162016.sau3gertosgr6mtu@skbuf>
Date:   Mon, 11 Apr 2022 19:20:16 +0300
From:   Vladimir Oltean <olteanv@...il.com>
To:     Nicolas Dichtel <nicolas.dichtel@...nd.com>
Cc:     Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Paolo Abeni <pabeni@...hat.com>
Subject: Re: What is the purpose of dev->gflags?

On Mon, Apr 11, 2022 at 06:10:49PM +0200, Nicolas Dichtel wrote:
> 
> Le 11/04/2022 à 17:49, Vladimir Oltean a écrit :
> > On Mon, Apr 11, 2022 at 05:43:01PM +0200, Nicolas Dichtel wrote:
> >>
> >> Le 11/04/2022 à 17:33, Vladimir Oltean a écrit :
> >> [snip]
> >>> Would you agree that the __dev_set_allmulti() -> __dev_notify_flags()
> >>> call path is dead code? If it is, is there any problem it should be
> >>> addressing which it isn't, or can we just delete it?
> >> I probably miss your point, why is it dead code?
> > 
> > Because __dev_set_allmulti() doesn't update dev->gflags, it means
> > dev->gflags == old_gflags. In turn, it means dev->gflags ^ old_gflags,
> > passed to "gchanges" of __dev_notify_flags(), is 0.
> I didn't take any assumptions on dev->gflags because two functions are called
> with dev as parameter (dev_change_rx_flags() and dev_set_rx_mode()).

You mean ops->ndo_change_rx_flags() or ops->ndo_set_rx_mode() are
expected to update dev->gflags?

> Even if __dev_notify_flags() is called with 0 for the last arg, it calls
> notifiers. Thus, this is not "dead code".

The relevant "changes" (dev->flags & old_flags) of the net_device which
may have changed from __dev_set_allmulti() are masked out from
call_netdevice_notifiers(), are they not?

	if (changes & IFF_UP) {
		/* doesn't apply */
	}

	if (dev->flags & IFF_UP &&
	    (changes & ~(IFF_UP | IFF_PROMISC | IFF_ALLMULTI | IFF_VOLATILE))) {
	               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
	               these changes are masked out