From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: correct locking in softirq Date: Fri, 27 Feb 2009 08:59:42 -0800 Message-ID: <20090227165942.GC6758@linux.vnet.ibm.com> References: <885896af0902262354v47ddf79ch3d62edaeb1e940bd@mail.gmail.com> <1235723346.4948.1352.camel@laptop> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Giacomo , linux-kernel@vger.kernel.org, netdev To: Peter Zijlstra Return-path: Received: from e4.ny.us.ibm.com ([32.97.182.144]:50435 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756004AbZB0Q7o (ORCPT ); Fri, 27 Feb 2009 11:59:44 -0500 Content-Disposition: inline In-Reply-To: <1235723346.4948.1352.camel@laptop> Sender: netdev-owner@vger.kernel.org List-ID: On Fri, Feb 27, 2009 at 09:29:06AM +0100, Peter Zijlstra wrote: > 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. If "softirq" also includes sections of local_bh_disable()ed code in process context, also including irq-disabled code, agreed! > The distinction between the two RCU variants is that the _bh variant can > have a slightly faster quiescent cycle. Especially when under heavy interrupt/softirq load. If a given CPU is totally consumed handling interrupts and softirqs in a non-CONFIG_PREEMPT_RT kernel, then the call_rcu() variant might never invoke its callback, while the call_rcu_bh() variant would still be able to do so in a timely fashion. The _bh() variant was inspired by simulate DoS attacks, work by Robert Olsson and Dipankar Sarma. Thanx, Paul