From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756384AbaCNWng (ORCPT ); Fri, 14 Mar 2014 18:43:36 -0400 Received: from merlin.infradead.org ([205.233.59.134]:54966 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755992AbaCNWne (ORCPT ); Fri, 14 Mar 2014 18:43:34 -0400 Date: Fri, 14 Mar 2014 23:43:17 +0100 From: Peter Zijlstra To: "Paul E. McKenney" Cc: Alexander Shishkin , Ingo Molnar , linux-kernel@vger.kernel.org, Frederic Weisbecker , Mike Galbraith , Paul Mackerras , Stephane Eranian , Andi Kleen Subject: Re: [PATCH] [RFC] perf: Fix a race between ring_buffer_detach() and ring_buffer_wakeup() Message-ID: <20140314224317.GQ30956@twins.programming.kicks-ass.net> References: <1394199526-6400-1-git-send-email-alexander.shishkin@linux.intel.com> <20140313195816.GJ21124@linux.vnet.ibm.com> <20140314095033.GP27965@twins.programming.kicks-ass.net> <20140314204736.GG21124@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140314204736.GG21124@linux.vnet.ibm.com> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 14, 2014 at 01:47:37PM -0700, Paul E. McKenney wrote: > This general idea can be made to work, but it will need some > internal-to-RCU help. One vulnerability of the patch below is the > following sequence of steps: > > 1. RCU has just finished a grace period, and is doing the > end-of-grace-period accounting. > > 2. The code below invokes rcu_batches_completed(). Let's assume > the result returned is 42. > > 3. RCU completes the end-of-grace-period accounting, and increments > rcu_sched_state.completed. > > 4. The code below checks ->rcu_batches against the result from > another invocation of rcu_batches_completed() and sees that > the 43 is not equal to 42, so skips the synchronize_rcu(). > > Except that a grace period has not actually completed. Boom!!! > > The problem is that rcu_batches_completed() is only intended to give > progress information on RCU. Ah, I thought I was missing something when I was looking through the rcu code in a hurry :-) I knew there'd be some subtlety between completed and gpnum and such :-) > What I can do is give you a pair of functions, one to take a snapshot of > the current grace-period state (returning an unsigned long) and another > to evaluate a previous snapshot, invoking synchronize_rcu() if there has > not been a full grace period in the meantime. > > The most straightforward approach would invoke acquiring the global > rcu_state ->lock on each call, which I am guessing just might be > considered to be excessive overhead. ;-) I should be able to decrease > the overhead to a memory barrier on each call, and perhaps even down > to an smp_load_acquire(). Accessing the RCU state probably costs you > a cache miss both times. > > Thoughts? Sounds fine, the attach isn't a hotpath, so even the locked version should be fine, but I won't keep you from making it all fancy and such :-)