Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755320Ab3JMULG (ORCPT ); Sun, 13 Oct 2013 16:11:06 -0400 Received: from order.stressinduktion.org ([87.106.68.36]:60247 "EHLO order.stressinduktion.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755103Ab3JMULE (ORCPT ); Sun, 13 Oct 2013 16:11:04 -0400 Date: Sun, 13 Oct 2013 22:11:02 +0200 From: Hannes Frederic Sowa To: "Paul E. McKenney" Cc: Mathieu Desnoyers , Eric Dumazet , Josh Triplett , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, 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 Subject: Re: [PATCH v2 tip/core/rcu 07/13] ipv6/ip6_tunnel: Apply rcu_access_pointer() to avoid sparse false positive Message-ID: <20131013201102.GD14021@order.stressinduktion.org> Mail-Followup-To: "Paul E. McKenney" , Mathieu Desnoyers , Eric Dumazet , Josh Triplett , linux-kernel@vger.kernel.org, mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, 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 References: <20131009225617.GH11709@jtriplet-mobl1> <20131010002833.GJ5790@linux.vnet.ibm.com> <20131010020422.GB24368@order.stressinduktion.org> <20131010190532.GQ5790@linux.vnet.ibm.com> <20131012022508.GA20321@order.stressinduktion.org> <20131012075336.GA5790@linux.vnet.ibm.com> <20131012164345.GB20321@order.stressinduktion.org> <20131012173734.GC20321@order.stressinduktion.org> <1882655271.38519.1381606938076.JavaMail.zimbra@efficios.com> <20131013111439.GE5790@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20131013111439.GE5790@linux.vnet.ibm.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2433 Lines: 53 On Sun, Oct 13, 2013 at 04:14:39AM -0700, Paul E. McKenney wrote: > On Sat, Oct 12, 2013 at 07:42:18PM +0000, Mathieu Desnoyers wrote: > > ----- Original Message ----- > > > On Sat, Oct 12, 2013 at 06:43:45PM +0200, Hannes Frederic Sowa wrote: > > > > Regarding the volatile access, I hope that the C11 memory model > > > > and enhancements to the compiler will some day provide a better > > > > way to express the semantics of what is tried to express here > > > > (__atomic_store_n/__atomic_load_n with the accompanied memory model, > > > > which could be even weaker to what a volatile access would enfore > > > > now and could guarantee atomic stores/loads). > > > > > > I just played around a bit more. Perhaps we could try to warn of silly > > > usages of ACCESS_ONCE(): > > > > > > --- a/include/linux/compiler.h > > > +++ b/include/linux/compiler.h > > > @@ -349,7 +349,11 @@ void ftrace_likely_update(struct ftrace_branch_data *f, > > > int val, int expect); > > > * use is to mediate communication between process-level code and irq/NMI > > > * handlers, all running on the same CPU. > > > */ > > > -#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x)) > > > +#define ACCESS_ONCE(x) (*({ \ > > > + compiletime_assert(sizeof(typeof(x)) <= sizeof(typeof(&x)), \ > > > + "ACCESS_ONCE likely not atomic"); \ > > > > AFAIU, ACCESS_ONCE() is not meant to ensure atomicity of load/store, > > but rather merely ensures that the compiler will not merge nor refetch > > accesses. I don't think the assert check you propose is appropriate with > > respect to the ACCESS_ONCE() semantic. > > I am with Mathieu on this one, at least unless there is some set of actual > bugs already in the kernel that these length checks would find. I guess my wording of "ACCESS_ONCE likely not atomic" was misplaced. Something like volatile access to memory larger than the processor register size is probably not what you intended. Use atomics or proper locking. ;) And maybe that is not even correct. > /me wonders about structs of size 3, 5, 6, and 7... Checked a x86_64 allyesconfig build with sizes above pointer size and odd parity and nothing broke. Greetings, Hannes -- 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/