From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra Subject: Re: correct locking in softirq Date: Fri, 27 Feb 2009 09:29:06 +0100 Message-ID: <1235723346.4948.1352.camel@laptop> References: <885896af0902262354v47ddf79ch3d62edaeb1e940bd@mail.gmail.com> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: linux-kernel@vger.kernel.org, netdev , "Paul E. McKenney" To: Giacomo Return-path: Received: from bombadil.infradead.org ([18.85.46.34]:57161 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753439AbZB0I3L (ORCPT ); Fri, 27 Feb 2009 03:29:11 -0500 In-Reply-To: <885896af0902262354v47ddf79ch3d62edaeb1e940bd@mail.gmail.com> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, 2009-02-27 at 08:54 +0100, Giacomo wrote: > Good morning > > Harald Welte's "The journey of a packet through the Linux 2.6.10 > network stack" article says that packet travelling inside > linux kernel 2.6 (the receive / input part) runs in softirq context. > > Hooking with netfilter's hooks in a kernel module, i need to read for > each packet received a list of rules. > > Since in input and prerouting hooks the context is softirq (perhaps > also in forward?), I need some read lock > feature. > > I currently use RCU lists and, while reading lists I use > > READ > > read_lock_bh() > > together with list_for_each_rcu() > > When changing, or flushing, rules, I use > > WRITE > > spin_lock() + list_add_tail_rcu() (adding) > > or spin_lock() + list_for_each_entry() (for listing and then freeing > with list_del_rcu() and call_rcu() ) > > The question is: > > - is the read part above correct? - do I really need _bh()? or should > I use simply read_lock() ? > > Thanks in advance rcu_read_lock() + call_rcu() are correct, even from softirq context, and mandatory if anything is exposed to anything other than softirq context. rcu_read_lock_bh() + call_rcu_bh() is usable IFF the data is only ever used from softirq. The distinction between the two RCU variants is that the _bh variant can have a slightly faster quiescent cycle.