netdev - Re: [PATCH next-net] ipv6: Use hlist_for_each_entry_rcu_bh() in ipv6_chk_same

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Message-ID: <CAJFZqHz1xFHaOus_LgKZi5Pgb0gW-c9uLvqZtS69G6vmqy+OCg@mail.gmail.com>
Date:	Tue, 1 Nov 2011 17:17:23 +0800
From:	RongQing Li <roy.qing.li@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: Re: [PATCH next-net] ipv6: Use hlist_for_each_entry_rcu_bh() in ipv6_chk_same_addr()

2011/11/1 David Miller <davem@...emloft.net>:
> From: RongQing Li <roy.qing.li@...il.com>
> Date: Tue, 1 Nov 2011 17:05:19 +0800
>
>> 2011/11/1 David Miller <davem@...emloft.net>:
>>> From: RongQing Li <roy.qing.li@...il.com>
>>> Date: Tue, 1 Nov 2011 16:33:49 +0800
>>>
>>>> Yes, But I think the code readable is not good,
>>>
>>> It is wasteful to add multiple BH disables and RCU memory
>>> barriers in code paths where it is not necessary.
>>>
>>> Your patch fixes no real bug, and adds a regression.
>>>
>>
>> hlist_for_each_entry_rcu_bh() does not disable BH,
>> it only check If BH is disabled.
>>
>> These codes is different from normal RCU convention.
>
> But adding _rcu does add memory barriers.
>

Yes,  But I hope we can keep this convention.

Like the below similar codes, which exists in everywhere,
If we  replace rcu_dereference_bh with dereference ..
it is not good solution, though we can reduce  memory barriers

	rcu_read_lock_bh();

	txq = dev_pick_tx(dev, skb);
	q = rcu_dereference_bh(txq->qdisc);
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