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Message-ID: <4AF0B6B9.2030707@gmail.com>
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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