linux-kernel - Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc

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Date:   Wed, 1 Sep 2021 18:13:14 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, linuxarm@...neuler.org,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        David Ahern <dsahern@...nel.org>
Subject: Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()

On Wed, Sep 1, 2021 at 5:47 PM Yunsheng Lin <linyunsheng@...wei.com> wrote:
>
> On 2021/9/1 18:39, Yunsheng Lin wrote:
> > Since tcp_tx_skb_cache is disabled by default in:
> > commit 0b7d7f6b2208 ("tcp: add tcp_tx_skb_cache sysctl")
> >
> > Add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb() to
> > avoid possible branch-misses.
> >
> > Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> > ---
> > Also, the sk->sk_tx_skb_cache may be both changed by allocation
> > and freeing side, I assume there may be some implicit protection
> > here too, such as the NAPI protection for rx?
>
> Hi, Eric
>    Is there any implicit protection for sk->sk_tx_skb_cache?
> As my understanding, sk_stream_alloc_skb() seems to be protected
> by lock_sock(), and the sk_wmem_free_skb() seems to be mostly
> happening in NAPI polling for TCP(when ack packet is received)
> without lock_sock(), so it seems there is no protection here?
>

Please look again.
This is protected by socket lock of course.
Otherwise sk_mem_uncharge() would be very broken, sk->sk_forward_alloc
is not an atomic field.

TCP stack has no direct relation  with NAPI.
It can run over loopback interface, no NAPI there.