From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH 2/2] ipvs: Use cond_resched_rcu_lock() helper when dumping connections Date: Fri, 26 Apr 2013 08:45:47 -0700 Message-ID: <20130426154547.GC3860@linux.vnet.ibm.com> References: <1366940708-10180-1-git-send-email-horms@verge.net.au> <1366940708-10180-3-git-send-email-horms@verge.net.au> <20130426080313.GC8669@dyad.programming.kicks-ass.net> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Simon Horman , Eric Dumazet , Julian Anastasov , Ingo Molnar , lvs-devel@vger.kernel.org, netdev@vger.kernel.org, netfilter-devel@vger.kernel.org, linux-kernel@vger.kernel.org, Pablo Neira Ayuso , Dipankar Sarma , dhaval.giani@gmail.com To: Peter Zijlstra Return-path: Content-Disposition: inline In-Reply-To: <20130426080313.GC8669@dyad.programming.kicks-ass.net> Sender: linux-kernel-owner@vger.kernel.org List-Id: netfilter-devel.vger.kernel.org On Fri, Apr 26, 2013 at 10:03:13AM +0200, Peter Zijlstra wrote: > On Fri, Apr 26, 2013 at 10:45:08AM +0900, Simon Horman wrote: > > > @@ -975,8 +975,7 @@ static void *ip_vs_conn_array(struct seq_file *seq, loff_t pos) > > return cp; > > } > > } > > - rcu_read_unlock(); > > - rcu_read_lock(); > > + cond_resched_rcu_lock(); > > } > > > While I agree with the sentiment I do find it a somewhat dangerous construct in > that it might become far too easy to keep an RCU reference over this break and > thus violate the RCU premise. > > Is there anything that can detect this? Sparse / cocinelle / smatch? If so it > would be great to add this to these checkers. I have done some crude coccinelle patterns in the past, but they are subject to false positives (from when you transfer the pointer from RCU protection to reference-count protection) and also false negatives (when you atomically increment some statistic unrelated to protection). I could imagine maintaining a per-thread count of the number of outermost RCU read-side critical sections at runtime, and then associating that counter with a given pointer at rcu_dereference() time, but this would require either compiler magic or an API for using a pointer returned by rcu_dereference(). This API could in theory be enforced by sparse. Dhaval Giani might have some ideas as well, adding him to CC. Thanx, Paul