netdev - Re: [PATCH net-next] net: sched: calls synchronize

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Message-ID: <CANn89iK7PzN6C4GXfwSasszdF1PyupR9xd7wsvoRiYrm0ARwtQ@mail.gmail.com>
Date: Fri, 10 Jan 2025 06:49:11 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Cong Wang <xiyou.wangcong@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Jamal Hadi Salim <jhs@...atatu.com>, Jiri Pirko <jiri@...nulli.us>, netdev@...r.kernel.org, 
	eric.dumazet@...il.com
Subject: Re: [PATCH net-next] net: sched: calls synchronize_net() only when needed

On Fri, Jan 10, 2025 at 6:33 AM Cong Wang <xiyou.wangcong@...il.com> wrote:
>
> On Thu, Jan 09, 2025 at 05:18:50PM +0000, Eric Dumazet wrote:
> > dev_deactivate_many() role is to remove the qdiscs
> > of a network device.
> >
> > When/if a qdisc is dismantled, an rcu grace period
> > is needed to make sure all outstanding qdisc enqueue
> > are done before we proceed with a qdisc reset.
> >
> > Most virtual devices do not have a qdisc (if we exclude
> > noqueue ones).
>
> Such as? To me, most virtual devices use noqueue:
>
> $ git grep IFF_NO_QUEUE -- drivers/net/
> drivers/net/amt.c:      dev->priv_flags         |= IFF_NO_QUEUE;
> drivers/net/bareudp.c:  dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/bonding/bond_main.c:        bond_dev->priv_flags |= IFF_BONDING | IFF_UNICAST_FLT | IFF_NO_QUEUE;
> drivers/net/caif/caif_serial.c: dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/dummy.c:    dev->priv_flags |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
> drivers/net/ethernet/chelsio/cxgb4/cxgb4_main.c:        dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/ethernet/netronome/nfp/nfp_net_repr.c:      netdev->priv_flags |= IFF_NO_QUEUE | IFF_DISABLE_NETPOLL;
> drivers/net/geneve.c:   dev->priv_flags |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
> drivers/net/gtp.c:      dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/ipvlan/ipvlan_main.c:       dev->priv_flags |= IFF_UNICAST_FLT | IFF_NO_QUEUE;
> drivers/net/ipvlan/ipvtap.c:    dev->priv_flags &= ~IFF_NO_QUEUE;
> drivers/net/loopback.c: dev->priv_flags         |= IFF_LIVE_ADDR_CHANGE | IFF_NO_QUEUE;
> drivers/net/macsec.c:   dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/macvlan.c:  dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/net_failover.c:     failover_dev->priv_flags |= IFF_UNICAST_FLT | IFF_NO_QUEUE;
> drivers/net/netdevsim/netdev.c:                    IFF_NO_QUEUE;
> drivers/net/netkit.c:   dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/nlmon.c:    dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/pfcp.c:     dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/team/team_core.c:   dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/veth.c:     dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/vrf.c:      dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/vsockmon.c: dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/vxlan/vxlan_core.c: dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/wan/hdlc_fr.c:      dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/wireguard/device.c: dev->priv_flags |= IFF_NO_QUEUE;
> drivers/net/wireless/virtual/mac80211_hwsim.c:  dev->priv_flags |= IFF_NO_QUEUE;
>
>
> And noqueue_qdisc_ops sets ->enqueue to noop_enqueue():
>
> struct Qdisc_ops noqueue_qdisc_ops __read_mostly = {
>         .id             =       "noqueue",
>         .priv_size      =       0,
>         .init           =       noqueue_init,
>         .enqueue        =       noop_enqueue,
>         .dequeue        =       noop_dequeue,
>         .peek           =       noop_dequeue,
>         .owner          =       THIS_MODULE,
> };

Sure, but please a look at :

static int noqueue_init(struct Qdisc *qdisc, struct nlattr *opt,
struct netlink_ext_ack *extack)
{
        /* register_qdisc() assigns a default of noop_enqueue if unset,
        * but __dev_queue_xmit() treats noqueue only as such
        * if this is NULL - so clear it here. */
        qdisc->enqueue = NULL;
        return 0;
}