Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756788Ab3JIXR7 (ORCPT ); Wed, 9 Oct 2013 19:17:59 -0400 Received: from mail-pd0-f178.google.com ([209.85.192.178]:52268 "EHLO mail-pd0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756615Ab3JIXR6 (ORCPT ); Wed, 9 Oct 2013 19:17:58 -0400 Message-ID: <1381360675.4971.45.camel@edumazet-glaptop.roam.corp.google.com> Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive From: Eric Dumazet To: Josh Triplett Cc: paulmck@linux.vnet.ibm.com, 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 Date: Wed, 09 Oct 2013 16:17:55 -0700 In-Reply-To: <20131009225617.GH11709@jtriplet-mobl1> References: <20131009212920.GA15413@linux.vnet.ibm.com> <1381354186-16285-1-git-send-email-paulmck@linux.vnet.ibm.com> <1381354186-16285-7-git-send-email-paulmck@linux.vnet.ibm.com> <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> Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.2.3-0ubuntu6 Content-Transfer-Encoding: 7bit Mime-Version: 1.0 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1138 Lines: 37 On Wed, 2013-10-09 at 15:56 -0700, Josh Triplett wrote: > I'd be really hesitant to introduce that type of direct assignment to an > __rcu pointer without wrapping it in some appropriately named macro, or > at the very least adding a comment. Well, there is no special magic here, in this specific case : - deleting an item in an rcu list Check list_del_rcu(), and you'll notice there is no _barrier_ Adding correct barriers is good, but please do not add them when not needed. It makes code hard to understand. ACCESS(*ptr) = value; is clear and autodocumented, because it highlights the potential problem, that is *ptr can be read without any barrier from another cpu. So we ask the compiler to not write temporary garbage in it. rcu_assign_pointer(*ptr, rcu_access_pointer(value)) is very confusing, because it hides the _real_ problem and add defensive programming tricks. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/