netdev - Re: [PATCH net-next 1/4] net: dev: Remove the preempt_disable() in netif_rx

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Date:   Wed, 2 Feb 2022 09:10:10 -0800
From:   Eric Dumazet <edumazet@...gle.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     bpf <bpf@...r.kernel.org>, netdev <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH net-next 1/4] net: dev: Remove the preempt_disable() in netif_rx_internal().

On Wed, Feb 2, 2022 at 4:28 AM Sebastian Andrzej Siewior
<bigeasy@...utronix.de> wrote:
>
> The preempt_disable() and rcu_disable() section was introduced in commit
>    bbbe211c295ff ("net: rcu lock and preempt disable missing around generic xdp")
>
> The backtrace shows that bottom halves were disabled and so the usage of
> smp_processor_id() would not trigger a warning.
> The "suspicious RCU usage" warning was triggered because
> rcu_dereference() was not used in rcu_read_lock() section (only
> rcu_read_lock_bh()). A rcu_read_lock() is sufficient.
>
> Remove the preempt_disable() statement which is not needed.

I am confused by this changelog/analysis of yours.

According to git blame, you are reverting this patch.

commit cece1945bffcf1a823cdfa36669beae118419351
Author: Changli Gao <xiaosuo@...il.com>
Date:   Sat Aug 7 20:35:43 2010 -0700

    net: disable preemption before call smp_processor_id()

    Although netif_rx() isn't expected to be called in process context with
    preemption enabled, it'd better handle this case. And this is why get_cpu()
    is used in the non-RPS #ifdef branch. If tree RCU is selected,
    rcu_read_lock() won't disable preemption, so preempt_disable() should be
    called explictly.

    Signed-off-by: Changli Gao <xiaosuo@...il.com>
    Signed-off-by: David S. Miller <davem@...emloft.net>


But I am not sure we can.

Here is the code in larger context:

#ifdef CONFIG_RPS
    if (static_branch_unlikely(&rps_needed)) {
        struct rps_dev_flow voidflow, *rflow = &voidflow;
        int cpu;

        preempt_disable();
        rcu_read_lock();

        cpu = get_rps_cpu(skb->dev, skb, &rflow);
        if (cpu < 0)
            cpu = smp_processor_id();

        ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);

        rcu_read_unlock();
        preempt_enable();
    } else
#endif

This code needs the preempt_disable().


>
> Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> ---
>  net/core/dev.c | 2 --
>  1 file changed, 2 deletions(-)
>
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1baab07820f65..325b70074f4ae 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -4796,7 +4796,6 @@ static int netif_rx_internal(struct sk_buff *skb)
>                 struct rps_dev_flow voidflow, *rflow = &voidflow;
>                 int cpu;
>
> -               preempt_disable();
>                 rcu_read_lock();
>
>                 cpu = get_rps_cpu(skb->dev, skb, &rflow);
> @@ -4806,7 +4805,6 @@ static int netif_rx_internal(struct sk_buff *skb)
>                 ret = enqueue_to_backlog(skb, cpu, &rflow->last_qtail);
>
>                 rcu_read_unlock();
> -               preempt_enable();
>         } else
>  #endif
>         {
> --
> 2.34.1
>