netdev - [QUESTION] Doubt about NAPI_GRO_CB(skb)->is

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Date:   Wed, 20 Dec 2017 17:09:40 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     <alexander.duyck@...il.com>,
        "davem@...emloft.net" <davem@...emloft.net>,
        "linuxarm@...wei.com" <linuxarm@...wei.com>,
        <yuxiaowu@...ilicon.com>, <wzhen.wang@...ilicon.com>,
        Xuehuahu <xuehuahu@...ilicon.com>
Subject: [QUESTION] Doubt about NAPI_GRO_CB(skb)->is_atomic in tcpv4 gro
 process

Hi, all
	I have some doubt about NAPI_GRO_CB(skb)->is_atomic when
analyzing the tcpv4 gro process:

Firstly we set NAPI_GRO_CB(skb)->is_atomic to 1 in dev_gro_receive:
https://elixir.free-electrons.com/linux/v4.15-rc4/source/net/core/dev.c#L4838

And then in inet_gro_receive, we check the NAPI_GRO_CB(skb)->is_atomic
before setting NAPI_GRO_CB(skb)->is_atomic according to IP_DF bit in the ip header:
https://elixir.free-electrons.com/linux/v4.15-rc4/source/net/ipv4/af_inet.c#L1319

struct sk_buff **inet_gro_receive(struct sk_buff **head, struct sk_buff *skb)
{
.....................
	for (p = *head; p; p = p->next) {
........................

		/* If the previous IP ID value was based on an atomic
		 * datagram we can overwrite the value and ignore it.
		 */
		if (NAPI_GRO_CB(skb)->is_atomic)                      //we check it here
			NAPI_GRO_CB(p)->flush_id = flush_id;
		else
			NAPI_GRO_CB(p)->flush_id |= flush_id;
	}

	NAPI_GRO_CB(skb)->is_atomic = !!(iph->frag_off & htons(IP_DF));  //we set it here
	NAPI_GRO_CB(skb)->flush |= flush;
	skb_set_network_header(skb, off);
................................
}

My question is whether we should check the NAPI_GRO_CB(skb)->is_atomic or NAPI_GRO_CB(p)->is_atomic?
If we should check NAPI_GRO_CB(skb)->is_atomic, then maybe it is unnecessary because it is alway true.
If we should check NAPI_GRO_CB(p)->is_atomic, maybe there is a bug here.

So what is the logic here? I am just start analyzing the gro, maybe I miss something obvious here.