netdev - Re: [patch] timers: add mod_timer

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Date:	Wed, 18 Feb 2009 13:33:14 +0100
From:	Patrick McHardy <kaber@...sh.net>
To:	Ingo Molnar <mingo@...e.hu>
CC:	Oleg Nesterov <oleg@...hat.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Stephen Hemminger <shemminger@...tta.com>,
	David Miller <davem@...emloft.net>,
	Rick Jones <rick.jones2@...com>,
	Eric Dumazet <dada1@...mosbay.com>, netdev@...r.kernel.org,
	netfilter-devel@...r.kernel.org, tglx@...utronix.de,
	Martin Josefsson <gandalf@...g.westbo.se>
Subject: Re: [patch] timers: add mod_timer_pending()

Ingo Molnar wrote:
> * Patrick McHardy <kaber@...sh.net> wrote:
> 
>> We need to avoid having a timer that was deleted by one CPU
>> getting re-added by another, but want to avoid taking the
>> conntrack lock for every timer update. The timer-internal
>> locking is enough for this as long as we have a mod_timer
>> variant that forwards a timer, but doesn't activate it in
>> case it isn't active already.
> 
> that makes sense - but the implementation is still somewhat 
> ugly. How about the one below instead? Not tested.

This seems to fulfill our needs. I also like the mod_timer_pending()
name better than mod_timer_noact().

> One open question is this construct in mod_timer():
> 
> +	/*
> +	 * This is a common optimization triggered by the
> +	 * networking code - if the timer is re-modified
> +	 * to be the same thing then just return:
> +	 */
> +	if (timer->expires == expires && timer_pending(timer))
> +		return 1;
> 
> We've had this for ages, but it seems rather SMP-unsafe. 
> timer_pending(), if used in an unserialized fashion, can be any 
> random value in theory - there's no internal serialization here 
> anywhere.
> 
> We could end up with incorrectly not re-activating a timer in 
> mod_timer() for example - have such things never been observed 
> in practice?

Yes, it seems racy if done for timers that might get activated.
For forwarding only without activation it seems OK, in that case
the timer_pending check doesn't seem necessary at all.

> So the original patch which added this to mod_timer_noact() was 
> racy i think, and we cannot preserve this optimization outside 
> of the timer list lock. (we could do it inside of it.)
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