From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758320AbXITRbr (ORCPT ); Thu, 20 Sep 2007 13:31:47 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754465AbXITRbi (ORCPT ); Thu, 20 Sep 2007 13:31:38 -0400 Received: from ug-out-1314.google.com ([66.249.92.168]:1709 "EHLO ug-out-1314.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754068AbXITRbh (ORCPT ); Thu, 20 Sep 2007 13:31:37 -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=pv9nV2V1OzHLo6sO/8siumIyitln+jiApJxcYxWq49ySdvpcXqhiiuJQ+xzSJXERcBnvpn5w6GSXlr1gokcllxeY0VnPa89uaC0hav4a+dDnYQwiek+aOV7Ls4XXZ2ravQRBtdxhj6vPOnXkKKFB6Q28kyYN1XFRgN1S38KDxiQ= Message-ID: Date: Thu, 20 Sep 2007 13:31:35 -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: <20070919234700.3b532dcd@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> <20070919174857.GA11922@linux.vnet.ibm.com> <20070919214110.5d832f28@lappy> <20070919221349.2935f69d@lappy> <20070919231943.4b121361@lappy> <20070919234700.3b532dcd@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 17:29:09 -0400 "Dmitry Torokhov" > wrote: > > > On 9/19/07, Peter Zijlstra wrote: > > > On Wed, 19 Sep 2007 16:41:04 -0400 "Dmitry Torokhov" > > > wrote: > > > > > > > > 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." > > > > > > That still does as it says in -rt. Its just that the interrupt handler > > > will be preemptible so the guarantees it gives are useless. > > > > Please note "... including NMI and hardware-interrupt handlers ..." > > -rt doesn't run interrupt handlers in hardware irq context anymore. OK, then what is the purpose of synchronize_sched() in -rt? You really need to provide users with a replacement. There are several drivers that use it and for example r8169 is not what you'd call a 'low performer'. I guess I can switch i8042 to use synchronize_irq(). That still works in -rt, doesn't it? That still leaves atkbd... > > > > > > > > > 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? > > > > > > sure, but the cache eviction caused by running the driver will have > > > more impact than the added rcu_read_{,un}lock() calls. > > > > Are you saying that adding rcu_read_{,un}lock() will help with cache > > eviction? How? > > No, I'm saying that its noise compared to the cache eviction overhead > it causes for others. > What about udelay(10)? It is probably also a noise but we shoudl not go and sprinkle it through drivers, should we? ;) -- Dmitry