netdev - Re: [PATCH net-next v2 1/3] net: dev: Remove preempt_disable() and get_cpu() in netif_rx

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Message-ID: <87bkzmb3j8.fsf@toke.dk>
Date:   Sat, 05 Feb 2022 00:44:27 +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 1/3] net: dev: Remove preempt_disable() and
 get_cpu() in netif_rx_internal().

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

> The preempt_disable() () section was introduced in commit
>     cece1945bffcf ("net: disable preemption before call smp_processor_id()")
>
> and adds it in case this function is invoked from preemtible context and
> because get_cpu() later on as been added.
>
> The get_cpu() usage was added in commit
>     b0e28f1effd1d ("net: netif_rx() must disable preemption")
>
> because ip_dev_loopback_xmit() invoked netif_rx() with enabled preemption
> causing a warning in smp_processor_id(). The function netif_rx() should
> only be invoked from an interrupt context which implies disabled
> preemption. The commit
>    e30b38c298b55 ("ip: Fix ip_dev_loopback_xmit()")
>
> was addressing this and replaced netif_rx() with in netif_rx_ni() in
> ip_dev_loopback_xmit().
>
> Based on the discussion on the list, the former patch (b0e28f1effd1d)
> should not have been applied only the latter (e30b38c298b55).
>
> Remove get_cpu() and preempt_disable() since the function is supposed to
> be invoked from context with stable per-CPU pointers. Bottom halves have
> to be disabled at this point because the function may raise softirqs
> which need to be processed.
>
> Link: https://lkml.kernel.org/r/20100415.013347.98375530.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>