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: Thu, 10 Oct 2013 12:05:32 -0700 Message-ID: <20131010190532.GQ5790@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> <20131010020422.GB24368@order.stressinduktion.org> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii To: Eric Dumazet , Josh Triplett , 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 Return-path: Content-Disposition: inline In-Reply-To: <20131010020422.GB24368@order.stressinduktion.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Thu, Oct 10, 2013 at 04:04:22AM +0200, Hannes Frederic Sowa 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. > > Interesting thread! > > Sorry to chime in and asking a question: > > Why do we need an ACCESS_ONCE here if rcu_assign_pointer can do without one? > In other words I wonder why rcu_assign_pointer is not a static inline function > to use the sequence point in argument evaluation (if I remember correctly this > also holds for inline functions) to not allow something like this: > > E.g. we want to publish which lock to take first to prevent an ABBA problem > (extreme example): > > rcu_assign_pointer(lockptr, min(lptr1, lptr2)); > > Couldn't a compiler spill the lockptr memory location as a temporary buffer > if the compiler is under register pressure? (yes, this seems unlikely if we > flushed out most registers to memory because of the barrier, but still... ;) ) > > This seems to be also the case if we publish a multi-dereferencing pointers > e.g. ptr->ptr->ptr. IIRC, sequence points only confine volatile accesses. For non-volatile accesses, the so-called "as-if rule" allows compiler writers to do some surprisingly global reordering. The reason that rcu_assign_pointer() isn't an inline function is because it needs to be type-generic, in other words, it needs to be OK to use it on any type of pointers as long as the C types of the two pointers match (the sparse types can vary a bit). One of the reasons for wanting a volatile cast in rcu_assign_pointer() is to prevent compiler mischief such as you described in your last two paragraphs. That said, it would take a very brave compiler to pull a pointer-referenced memory location into a register and keep it there. Unfortunately, increasing compiler bravery seems to be a solid long-term trend. Thanx, Paul