netdev - Re: [PATCH net-next v2 2/3] net: dev: Makes sure netif

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Date:   Sat, 05 Feb 2022 00:44:53 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        bpf@...r.kernel.org, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH net-next v2 2/3] net: dev: Makes sure netif_rx() can be
 invoked in any context.

Sebastian Andrzej Siewior <bigeasy@...utronix.de> writes:

> Dave suggested a while ago (eleven years by now) "Let's make netif_rx()
> work in all contexts and get rid of netif_rx_ni()". Eric agreed and
> pointed out that modern devices should use netif_receive_skb() to avoid
> the overhead.
> In the meantime someone added another variant, netif_rx_any_context(),
> which behaves as suggested.
>
> netif_rx() must be invoked with disabled bottom halves to ensure that
> pending softirqs, which were raised within the function, are handled.
> netif_rx_ni() can be invoked only from process context (bottom halves
> must be enabled) because the function handles pending softirqs without
> checking if bottom halves were disabled or not.
> netif_rx_any_context() invokes on the former functions by checking
> in_interrupts().
>
> netif_rx() could be taught to handle both cases (disabled and enabled
> bottom halves) by simply disabling bottom halves while invoking
> netif_rx_internal(). The local_bh_enable() invocation will then invoke
> pending softirqs only if the BH-disable counter drops to zero.
>
> Eric is concerned about the overhead of BH-disable+enable especially in
> regard to the loopback driver. As critical as this driver is, it will
> receive a shortcut to avoid the additional overhead which is not needed.
>
> Add a local_bh_disable() section in netif_rx() to ensure softirqs are
> handled if needed. Provide the internal bits as __netif_rx() which can
> be used by the loopback driver. This function is not exported so it
> can't be used by modules.
> Make netif_rx_ni() and netif_rx_any_context() invoke netif_rx() so they
> can be removed once they are no more users left.
>
> Link: https://lkml.kernel.org/r/20100415.020246.218622820.davem@davemloft.net
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>

Reviewed-by: Toke Høiland-Jørgensen <toke@...hat.com>