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: Sun, 13 Oct 2013 04:14:39 -0700 Message-ID: <20131013111439.GE5790@linux.vnet.ibm.com> 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> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Hannes Frederic Sowa , 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 To: Mathieu Desnoyers Return-path: Content-Disposition: inline In-Reply-To: <1882655271.38519.1381606938076.JavaMail.zimbra@efficios.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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. /me wonders about structs of size 3, 5, 6, and 7... Thanx, Paul > Thanks, > > Mathieu > > > + (volatile typeof(x) *)&(x); \ > > +})) > > > > /* Ignore/forbid kprobes attach on very low level functions marked by this > > attribute: */ > > #ifdef CONFIG_KPROBES > > > > > > -- > Mathieu Desnoyers > EfficiOS Inc. > http://www.efficios.com >