From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753692Ab1AUVWv (ORCPT ); Fri, 21 Jan 2011 16:22:51 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:49315 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753073Ab1AUVWt (ORCPT ); Fri, 21 Jan 2011 16:22:49 -0500 Date: Fri, 21 Jan 2011 13:22:45 -0800 From: "Paul E. McKenney" To: Eric Dumazet Cc: Nick Piggin , Jeff Moyer , Jan Kara , Andrew Morton , linux-fsdevel , linux-kernel@vger.kernel.org Subject: Re: [patch] fs: aio fix rcu lookup Message-ID: <20110121212245.GN17752@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20110120040308.GD8476@linux.vnet.ibm.com> <20110120200237.GC17752@linux.vnet.ibm.com> <1295554555.2613.15.camel@edumazet-laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <1295554555.2613.15.camel@edumazet-laptop> User-Agent: Mutt/1.5.20 (2009-06-14) X-Content-Scanned: Fidelis XPS MAILER Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Jan 20, 2011 at 09:15:55PM +0100, Eric Dumazet wrote: > Le jeudi 20 janvier 2011 à 12:02 -0800, Paul E. McKenney a écrit : > > On Fri, Jan 21, 2011 at 05:31:53AM +1100, Nick Piggin wrote: > > > call_rcu() is the obvious alternative, yes. > > > > > > Basically, once we give in to synchronize_rcu() we're basically giving > > > up. That's certainly a very good tradeoff for something like filesystem > > > unregistration or module unload, it buys big simplifications in real > > > fastpaths. But I just don't think it should be taken lightly. > > > > Makes sense to me! > > > > BTW, on your earlier usage classification: > > > > > I think synchronize_rcu should firstly not be used unless it gives a good > > > simplification, or speedup in fastpath. > > > > > > When that is satified, then it is a question of exactly what kind of slow > > > path it should be used in. I don't think it should be used in process- > > > synchronous code (eg syscalls) except for error cases, resource > > > exhaustion, management syscalls (like module unload). > > > > I don't have any feedback either way on your guidance to where > > synchronize_rcu() should be used, as I believe that it depends a lot > > on the details of usage, and would vary from one part of the kernel > > to another, and possibly also over time. > > > > Sometime, a mixture of call_rcu() and synchronize_rcu() is used, to have > a limit on pending callbacks (eating too much memory) > > net/ipv4/fib_trie.c for example issues call_rcu() most of the time, but > is able to trigger one synchronize_rcu() if more than XXX (128) pages of > memory were queued in rcu queues. > > For details, check commit c3059477fce2d956 > (ipv4: Use synchronize_rcu() during trie_rebalance()) Good point! Thanx, Paul