From mboxrd@z Thu Jan 1 00:00:00 1970 From: Josh Triplett Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive Date: Thu, 10 Oct 2013 17:20:44 -0700 Message-ID: <20131011002044.GA32546@jtriplet-mobl1> References: <1381354949.4971.20.camel@edumazet-glaptop.roam.corp.google.com> <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> 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: "Paul E. McKenney" Return-path: Content-Disposition: inline In-Reply-To: <20131010002833.GJ5790@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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. 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). - Josh Triplett