From mboxrd@z Thu Jan 1 00:00:00 1970 From: Luciano Coelho Subject: Re: [PATCH v3] netfilter: Xtables: idletimer target implementation Date: Thu, 10 Jun 2010 15:42:13 +0300 Message-ID: <1276173733.5453.1.camel@chilepepper> References: <1275592445-15555-1-git-send-email-luciano.coelho@nokia.com> <4C0F9B17.10504@trash.net> <1276096275.31892.117.camel@chilepepper> <1276105707.11199.12.camel@powerslave> <1276108939.11199.23.camel@powerslave> <4C10B954.9080603@trash.net> Mime-Version: 1.0 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit Cc: ext Jan Engelhardt , "netfilter-devel@vger.kernel.org" , "netdev@vger.kernel.org" , Timo Teras To: ext Patrick McHardy Return-path: In-Reply-To: <4C10B954.9080603@trash.net> Sender: netdev-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org On Thu, 2010-06-10 at 12:07 +0200, ext Patrick McHardy wrote: > Luciano Coelho wrote: > > On Wed, 2010-06-09 at 19:48 +0200, Coelho Luciano (Nokia-D/Helsinki) > > wrote: > > > >> On Wed, 2010-06-09 at 17:18 +0200, ext Jan Engelhardt wrote: > >> > >>>>>> + 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! > >> > > > > Hmmm... after struggling with this for a while, I think it's not really > > possible to simply create the sysfs file outside of the lock, because if > > the sysfs creation fails, we will again risk a race condition. > > > > I think the only way is to delay the sysfs file creation and do it in a > > workqueue. > > > > Why don't you simply use a mutex instead of the spinlock? It would be better > to only do the lookup once and store the timer pointer in the target > structure > anyways. Wow! Again I have been totally blind and focusing only in a solution for the spinlock problem, while using a mutex would ease things up quite a lot! Thanks for the suggestion, I'll re-spin my patch (pun intended?) with a mutex. I also agree that it makes more sense to lookup and store the timer in the targe. -- Cheers, Luca.