From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756799AbXISUlU (ORCPT ); Wed, 19 Sep 2007 16:41:20 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751036AbXISUlH (ORCPT ); Wed, 19 Sep 2007 16:41:07 -0400 Received: from nf-out-0910.google.com ([64.233.182.188]:22915 "EHLO nf-out-0910.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751338AbXISUlF (ORCPT ); Wed, 19 Sep 2007 16:41:05 -0400 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=beta; h=received:message-id:date:from:to:subject:cc:in-reply-to:mime-version:content-type:content-transfer-encoding:content-disposition:references; b=HRB2PXUX6IhHbBsT6dS5aMLHWAcjgsagCC4Z/gG/UgWiFSTjPlnWxfIXgCS7lu0VUOgbR03s/xaXPO2JDoyHju7HjfXE98BNip6pyHV+Zss2gPJJz9iE/dVHvHfcbpDZrKA37WBIswMM7GE1zaoQQbBLpPxnZuH7LiLPgAF+ehQ= Message-ID: Date: Wed, 19 Sep 2007 16:41:04 -0400 From: "Dmitry Torokhov" To: "Peter Zijlstra" Subject: Re: [RFC][PATCH 2/6] lockdep: validate rcu_dereference() vs rcu_read_lock() Cc: paulmck@linux.vnet.ibm.com, linux-kernel@vger.kernel.org, "Ingo Molnar" , "Andrew Morton" , "Nick Piggin" In-Reply-To: <20070919221349.2935f69d@lappy> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Content-Disposition: inline References: <20070919104125.286538000@chello.nl> <20070919153750.GB8666@linux.vnet.ibm.com> <20070919173249.GE8666@linux.vnet.ibm.com> <20070919174857.GA11922@linux.vnet.ibm.com> <20070919214110.5d832f28@lappy> <20070919221349.2935f69d@lappy> Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On 9/19/07, Peter Zijlstra wrote: > 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. And the IRQ handler needs to do some extra job... Anyway, it looks -rt breaks synchronize_sched() and needs to have it fixed: "/** * synchronize_sched - block until all CPUs have exited any non-preemptive * kernel code sequences. * * This means that all preempt_disable code sequences, including NMI and * hardware-interrupt handlers, in progress on entry will have completed * before this primitive returns." > > 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. Please realize that evey microsecond wasted on a 'low performant' driver is taken from high performers and if we can help it why shouldn't we? -- Dmitry