From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755274AbXISUP2 (ORCPT ); Wed, 19 Sep 2007 16:15:28 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1750935AbXISUPS (ORCPT ); Wed, 19 Sep 2007 16:15:18 -0400 Received: from pentafluge.infradead.org ([213.146.154.40]:38629 "EHLO pentafluge.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750758AbXISUPQ (ORCPT ); Wed, 19 Sep 2007 16:15:16 -0400 Date: Wed, 19 Sep 2007 22:13:49 +0200 From: Peter Zijlstra To: "Dmitry Torokhov" Cc: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, "Ingo Molnar" , "Andrew Morton" , "Nick Piggin" Subject: Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read_lock() Message-ID: <20070919221349.2935f69d@lappy> In-Reply-To: References: <20070919104125.286538000@chello.nl> <20070919163111.05c18021@twins> <20070919153750.GB8666@linux.vnet.ibm.com> <20070919173249.GE8666@linux.vnet.ibm.com> <20070919174857.GA11922@linux.vnet.ibm.com> <20070919214110.5d832f28@lappy> X-Mailer: Claws Mail 3.0.0 (GTK+ 2.11.6; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, 19 Sep 2007 15:49:24 -0400 "Dmitry Torokhov" wrote: > On 9/19/07, Peter Zijlstra wrote: > > On Wed, 19 Sep 2007 14:49:56 -0400 "Dmitry Torokhov" > > wrote: > > > > > On 9/19/07, Paul E. McKenney wrote: > > > > > > PS to previous -- any problem with inserting rcu_read_lock() and > > > > rcu_read_unlock() around the portion of the IRQ handler that has > > > > these accesses? > > > > > > > > > > I guess I could but it is an extra lock that needs to be managed and > > > given the fact that it is not really needed (other to make a newly > > > developed tool happy) I am hestsant to do that. > > > > As is, these sites are a bug in -rt and we'll need to fix them anyway. > > > > As for the code you pointed me to, the i8042 driver, it seems to play > > way to funny tricks for a simple 'slow' driver. > > Even "slow" driver should try not to slow down the rest of the system > if it can help it. I am sorry if the thing it does do not quite fit in > with the changes you are proposing but it does not make the exeisting > code invalid. > > > > > If you replace the spin_lock() + sync_sched(), with rcu_read_lock() + > > rcu_call() it should work again without adding an extra lock. > > > > Except that I need spin_lock_irq for other reasons. I could take the > same lock in write-side code and not use RCU at all but using RCU > allows opening/closing input devices without slowing down interrupt > handlers so why not use it? If the IRQ handler does rcu_read_lock(),unlock() and the i8042_stop() function does sync_rcu() instead of _sched(), it should be good again. It will not affect anything else than the task that calls _stop(). And even there the only change is that the sleep might be a tad longer. I find it curious that a driver that is 'low performant' and does not suffer lock contention pioneers locking schemes. I agree with optimizing, but this is not the place to push the envelope.