netdev - Re: [PATCH v3] netfilter: Xtables: idletimer target implementation

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Date:	Wed, 09 Jun 2010 20:48:27 +0300
From:	Luciano Coelho <luciano.coelho@...ia.com>
To:	ext Jan Engelhardt <jengelh@...ozas.de>
Cc:	ext Patrick McHardy <kaber@...sh.net>,
	"netfilter-devel@...r.kernel.org" <netfilter-devel@...r.kernel.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	Timo Teras <timo.teras@....fi>
Subject: Re: [PATCH v3] netfilter: Xtables: idletimer target implementation

Hi Jan,

On Wed, 2010-06-09 at 17:18 +0200, ext Jan Engelhardt wrote:
> On Wednesday 2010-06-09 17:11, Luciano Coelho wrote:
> >Do you think it's okay to leave it like this for now and extend it for
> >multiple namespace support with a future patch?
> 
> Yes. Least thing we need is one humongous patch. :)

Thanks! That was my concern too.  One huge and very complex patch is not
a nice thing to have. ;)


> >> > +	timer = __idletimer_tg_find_by_label(info->label);
> >> > +	if (!timer) {
> >> > +		spin_unlock(&list_lock);
> >> > +		timer = idletimer_tg_create(info);
> >> >   
> >> 
> >> How does this prevent creating the same timer twice?
> >
> >The timer will only be created if __idletimer_tg_find_by_label() returns
> >NULL, which means that no timer with that label has been found.  "info"
> >won't be the same if info->label is different, right? Or can it change
> >on the fly?
> 
> One thing to be generally aware about is that things could potentially
> be instantiated by another entity between the time a label was looked up
> with negative result and the time one tries to add it.
> It may thus be required to extend keeping the lock until after
> idletimer_tg_create, in other words, lookup and create must be atomic
> to the rest of the world.

Ahh, sure! I missed the actual point of Patrick's question.  I had the
idletimer_tg_create() inside the lock, but when I added the
sysfs_create_file() there (which can sleep), I screwed up with the
locking.

I'll move the sysfs file creation to outside that function so I can keep
the lock until after the timer is added to the list.  Thanks for
clarifying!


-- 
Cheers,
Luca.

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