netdev - Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

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Date:	Wed, 04 Nov 2009 00:03:21 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	William Allen Simpson <william.allen.simpson@...il.com>
CC:	Linux Kernel Developers <linux-kernel@...r.kernel.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next-2.6 PATCH RFC] TCPCT part 1d: generate Responder Cookie

William Allen Simpson a écrit :
> Eric Dumazet wrote:
>> This patch looks fine, but I dont see how this new function is used.
>>
>> Some points :
>>
>> 1) We are working hard to remove rwlocks from network stack, so please
>> dont
>> add a new one. You probably can use a seqlock or RCU, or a server
>> handling 10.000 connections request per second on many NIC will hit
>> this rwlock.
>>
> This is my attempt at using RCU, as seqlock didn't seem to apply (and is
> missing any Documentation.)
> 

That seems very good, thanks, we can sort out details later, when full picture
is available.

> After the discussion about context, one question that I have is the need
> for the _bh suffix?
> 
> +        rcu_read_lock_bh();
> +        memcpy(&xvp->cookie_bakery[0],
> +               &rcu_dereference(tcp_secret_generating)->secrets[0],
> +               sizeof(tcp_secret_generating->secrets));
> +        rcu_read_unlock_bh();
> 

Well, you dont need to disable BH in this code running in softirq context only.

Just use rcu_read_lock() (like you use spin_lock() in same function/context)


> 
> Documentation/RCU/checklist.txt #7 says:
> 
>   One exception to this rule: rcu_read_lock() and rcu_read_unlock()
>   may be substituted for rcu_read_lock_bh() and rcu_read_unlock_bh()
>   in cases where local bottom halves are already known to be
>   disabled, for example, in irq or softirq context.  Commenting
>   such cases is a must, of course!  And the jury is still out on
>   whether the increased speed is worth it.
> 

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