From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive Date: Fri, 11 Oct 2013 06:25:12 -0700 Message-ID: <20131011132512.GW5790@linux.vnet.ibm.com> References: <20131009215747.GA5790@linux.vnet.ibm.com> <1381356624.4971.26.camel@edumazet-glaptop.roam.corp.google.com> <20131009223652.GC5790@linux.vnet.ibm.com> <1381359077.4971.37.camel@edumazet-glaptop.roam.corp.google.com> <20131009225617.GH11709@jtriplet-mobl1> <1381360675.4971.45.camel@edumazet-glaptop.roam.corp.google.com> <20131009234040.GB14055@jtriplet-mobl1> <1381363960.4971.55.camel@edumazet-glaptop.roam.corp.google.com> <20131010002833.GJ5790@linux.vnet.ibm.com> <20131011002044.GA32546@jtriplet-mobl1> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Eric Dumazet , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, "David S. Miller" , Alexey Kuznetsov , James Morris , Hideaki YOSHIFUJI , Patrick McHardy , netdev@vger.kernel.org To: Josh Triplett Return-path: Content-Disposition: inline In-Reply-To: <20131011002044.GA32546@jtriplet-mobl1> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, Oct 10, 2013 at 05:20:44PM -0700, Josh Triplett wrote: > On Wed, Oct 09, 2013 at 05:28:33PM -0700, Paul E. McKenney wrote: > > On Wed, Oct 09, 2013 at 05:12:40PM -0700, Eric Dumazet wrote: > > > On Wed, 2013-10-09 at 16:40 -0700, Josh Triplett wrote: > > > > > > > that. Constructs like list_del_rcu are much clearer, and not > > > > open-coded. Open-coding synchronization code is almost always a Bad > > > > Idea. > > > > > > OK, so you think there is synchronization code. > > > > > > I will shut up then, no need to waste time. > > > > As you said earlier, we should at least get rid of the memory barrier > > as long as we are changing the code. > > > > Josh, what would you suggest as the best way to avoid the memory barrier, > > keep sparse happy, and not be too ugly? > > The more I think about it, the more I realize that assigning an __rcu > pointer to an __rcu pointer *without* a memory barrier is a sufficiently > uncommon case that you probably *should* just write an open-coded > assignment. Just please put a very clear comment right before it. Fair enough, will do! Given earlier email, I believe that Eric is fine with this, and if he isn't I am sure he will let us know. ;-) > I'd originally thought it might make sense to have a macro similar to > rcu_assign_pointer, but I just don't think this is a common enough case, > and we don't want people thinking they can use this in general for __rcu > to __rcu assignments (most of which still need a memory barrier). Yep, it is a rather small fraction of rcu_assign_pointer() instances. Thanx, Paul