From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH] md: do not use ++ in rcu_dereference() argument Date: Thu, 9 Sep 2010 20:46:03 -0700 Message-ID: <20100910034603.GA2612@linux.vnet.ibm.com> References: <1283711539-7123-1-git-send-email-segooon@gmail.com> <201009072200.59509.arnd@arndb.de> <20100907205015.GJ2448@linux.vnet.ibm.com> <201009091714.44013.arnd@arndb.de> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <201009091714.44013.arnd@arndb.de> Sender: linux-kernel-owner@vger.kernel.org To: Arnd Bergmann Cc: Kulikov Vasiliy , kernel-janitors@vger.kernel.org, Neil Brown , Jens Axboe , linux-raid@vger.kernel.org, linux-kernel@vger.kernel.org List-Id: linux-raid.ids On Thu, Sep 09, 2010 at 05:14:43PM +0200, Arnd Bergmann wrote: > On Tuesday 07 September 2010, Paul E. McKenney wrote: > > On Tue, Sep 07, 2010 at 10:00:58PM +0200, Arnd Bergmann wrote: > > > On Tuesday 07 September 2010 21:21:55 Kulikov Vasiliy wrote: > > > > #define __rcu_dereference_check(p, c, space) \ > > > > ({ \ > > > > typeof(*p) *_________p1 = (typeof(*p)*__force )ACCESS_ONCE(p); \ > > > > ^ > > > > rcu_lockdep_assert(c); \ > > > > (void) (((typeof (*p) space *)p) == p); \ > > > > ^ ^ > > > > smp_read_barrier_depends(); \ > > > > ((typeof(*p) __force __kernel *)(_________p1)); \ > > > > }) > > > > > > > > If I understand this, it is evaluated three times, right? > > > > > > Yes, that looks like my own fault, I added that :( > > > > > > This patch seems to fix it, but I need to think about it some more > > > to make sure it still does everything we need. > > > > Let me know when you are satisfied with it, and then I will pick it up. > > I guess it would be good to put it in now. I haven't had the time > to try out all cases, but the current code in -next is definitely > broken, so please put the fix in now. Hmmm... One approach would be have a secondary macro that was: #define __rcu_dereference_check_sparse(p, space) \ (void) (((typeof (*p) space *)p) == p); when running sparse and: #define __rcu_dereference_check_sparse(p, space) otherwise. Would that do the trick? Thanx, Paul