Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1759383AbXEOWBU (ORCPT ); Tue, 15 May 2007 18:01:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1755956AbXEOWBF (ORCPT ); Tue, 15 May 2007 18:01:05 -0400 Received: from mail.screens.ru ([213.234.233.54]:41780 "EHLO mail.screens.ru" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756063AbXEOWBE (ORCPT ); Tue, 15 May 2007 18:01:04 -0400 Date: Wed, 16 May 2007 02:00:24 +0400 From: Oleg Nesterov To: Tejun Heo Cc: Andrew Morton , David Chinner , David Howells , Gautham Shenoy , Jarek Poplawski , Ingo Molnar , Srivatsa Vaddagiri , linux-kernel@vger.kernel.org Subject: Re: [PATCH] make cancel_rearming_delayed_work() reliable Message-ID: <20070515220024.GA615@tv-sign.ru> References: <20070503204226.GA212@tv-sign.ru> <464475EB.3000408@gmail.com> <20070511134714.GA191@tv-sign.ru> <46448965.7070500@gmail.com> <20070511145345.GA240@tv-sign.ru> <4645559D.4050602@gmail.com> <20070513192753.GA3014@tv-sign.ru> <46477239.9030007@gmail.com> <20070514194446.GA159@tv-sign.ru> <46496EC1.3090109@gmail.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <46496EC1.3090109@gmail.com> User-Agent: Mutt/1.5.11 Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2259 Lines: 69 On 05/15, Tejun Heo wrote: > > Oleg Nesterov wrote: > > > So, try_to_grab_pending() should check "VALID && pointers equal" atomically. > > We can't do "if (VALID && cwq == get_wq_data(work))". We should do something > > like this > > > > (((long)cwq) | VALID | PENDING) == atomic_long_read(&work->data) > > > > Yes? I need to think more about this. > > I don't think the test for PENDING should be atomic too. cwq pointer > and VALID is one package. PENDING lives its own life as a atomic bit > switch. Yes sure, it should not be atomic. But (VALID && !PENDING) == BUG, so we can't just "kill" PENDING form the check above. > > BTW, in _theory_, spinlock() is not a read barrier, yes? > > It actually is. > > > From Documentation/memory-barriers.txt > > > > Memory operations that occur before a LOCK operation may appear to happen > > after it completes. > > Which means that spin_lock() isn't a write barrier. yes, it is not very clear which "Memory operations" memory-barriers.txt describes. > lock is read > barrier, unlock is write barrier. (To avoid a possible confusion: I am not arguing, I am trying to understand, and please also note "in _theory_" above) Is it so? Shoudn't we document this if it is true? > Otherwise, locking doesn't make much > sense. Why? Could you please give a code example we have which relies on this? > If we're going the barrier way, I think we're better off with > explicit smp_wmb(). It's only barrier() on x86/64. Yes. But note that we don't have any reason to do set_wq_data() under cwq->lock (this is also true for wake_up(->more_work) btw), so it makes sense to do this change anyway. And "wmb + spin_lock" looks a bit strange, I _suspect_ spin_lock() means a full barrier on most platforms. Could you also look at http://marc.info/?t=116275561700001&r=1 and, in particular, http://marc.info/?l=linux-kernel&m=116281136122456 Thanks! Oleg. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/