Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1030730AbWK3REs (ORCPT ); Thu, 30 Nov 2006 12:04:48 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1030742AbWK3REr (ORCPT ); Thu, 30 Nov 2006 12:04:47 -0500 Received: from e6.ny.us.ibm.com ([32.97.182.146]:682 "EHLO e6.ny.us.ibm.com") by vger.kernel.org with ESMTP id S1030736AbWK3REq (ORCPT ); Thu, 30 Nov 2006 12:04:46 -0500 Date: Thu, 30 Nov 2006 09:05:41 -0800 From: "Paul E. McKenney" To: Oleg Nesterov Cc: "Paul E. McKenney" , Andrew Morton , Jens Axboe , Alan Stern , Josh Triplett , linux-kernel@vger.kernel.org Subject: Re: [RFC, PATCH 1/2] qrcu: "quick" srcu implementation Message-ID: <20061130170541.GA1869@us.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20061129235303.GA1118@oleg> <20061130015714.GC1350@oleg> <20061130024621.GL2335@us.ibm.com> <20061130032252.GA4101@oleg> <20061130033757.GA4110@oleg> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20061130033757.GA4110@oleg> User-Agent: Mutt/1.4.1i Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1281 Lines: 34 On Thu, Nov 30, 2006 at 06:37:57AM +0300, Oleg Nesterov wrote: > On 11/30, Oleg Nesterov wrote: > > > > On 11/29, Paul E. McKenney wrote: > > > > > > Hmmm... Now I am wondering if the memory barriers inherent in the > > > __wait_event() suffice for this last barrier... :-/ Thoughts? > > > > > > > + smp_mb(); > > > > Fastpath skips __wait_event(), and it is possible that the reader does > > lock/unlock between the first 'mb()' and 'if (atomic_read() == 1)'. > > In fact, a slow path needs (I think) it too. We can have an unrelated > wakeup, and then the reader does unlock() before we check !atomic_read() > in the __wait_event()'s loop. The reader removes us from ->wq, in that > case finish_wait() does nothing. Good point -- I was forgetting about the fastpath checks in __wait_event(). How about something like this? /* * The following memory barrier is needed to perserve ordering * in the case where __wait_event() follows its fastpath, * which includes neither locks nor memory barriers. */ Thanx, Paul - 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/