linux-kernel - Re: [RFT 2/4] Add mod_timer

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Message-ID: <20090218215140.GA3505@elte.hu>
Date:	Wed, 18 Feb 2009 22:51:40 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	shemminger@...tta.com, kaber@...sh.net, rick.jones2@...com,
	dada1@...mosbay.com, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, tglx@...utronix.de,
	gandalf@...g.westbo.se, linux-kernel@...r.kernel.org
Subject: Re: [RFT 2/4] Add mod_timer_noact


* David Miller <davem@...emloft.net> wrote:

> From: Ingo Molnar <mingo@...e.hu>
> Date: Wed, 18 Feb 2009 12:01:44 +0100
> 
> > * David Miller <davem@...emloft.net> wrote:
> > 
> > | Introduce mod_timer_noact() which for example is to replace 
> > | the calls to del_timer()/add_timer() in 
> > | __nf_ct_refresh_acct(). It works like mod_timer() but doesn't 
> > | activate or modify the timeout of an inactive timer which is 
> > | the behaviour we want in order to be able to use timers as a 
> > | means of synchronization in nf_conntrack.
> > 
> > It does not mention the overhead to the regular timer interfaces 
> > at all, nor does it explain the reasons for this change 
> > adequately.
> 
> You (conveniently) skipped this part of his commit message, so
> I guess this is the part you didn't read very carefully:
> 
> 	A later patch will modify __nf_ct_refresh_acct() to use
> 	mod_timer_noact() which will then save one spin_lock_irqsave()
> 	/ spin_lock_irqrestore() pair per conntrack timer update. This
> 	will also get rid of the race we currently have without adding
> 	more locking in nf_conntrack.
> 
> The whole point is to avoid two spin_lock_irqsave() sequences, thus
> taking the timer locks twice.
> 
> So Ingo, when you say in response:
> 
> 	Why don't you use?
> 
> 		if (del_timer())
> 			add_timer();
> 
> you really look foolish and, in fact, disrespectful to Stephen.
> 
> This was my objection to your email, it proved that you didn't
> really read his changelog message.  He explained perfectly well
> what the final goal was of his changes.
> 
> And you have this knee-jerk reaction quite often.

You accusing me of knee-jerk reaction is the joke of the century 
;-)

Anyway, it's all handled, you just need to read the rest of the 
thread.

Thanks,

	Ingo
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