From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762592AbXISRtT (ORCPT ); Wed, 19 Sep 2007 13:49:19 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1751056AbXISRtF (ORCPT ); Wed, 19 Sep 2007 13:49:05 -0400 Received: from e3.ny.us.ibm.com ([32.97.182.143]:43028 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750839AbXISRtC (ORCPT ); Wed, 19 Sep 2007 13:49:02 -0400 Date: Wed, 19 Sep 2007 10:48:57 -0700 From: "Paul E. McKenney" To: Dmitry Torokhov Cc: Peter Zijlstra , 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: <20070919174857.GA11922@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20070919104125.286538000@chello.nl> <20070919105054.332308000@chello.nl> <20070919163111.05c18021@twins> <20070919153750.GB8666@linux.vnet.ibm.com> <20070919173249.GE8666@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20070919173249.GE8666@linux.vnet.ibm.com> User-Agent: Mutt/1.5.13 (2006-08-11) Sender: linux-kernel-owner@vger.kernel.org X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Sep 19, 2007 at 10:32:49AM -0700, Paul E. McKenney wrote: > On Wed, Sep 19, 2007 at 12:59:10PM -0400, Dmitry Torokhov wrote: > > On 9/19/07, Paul E. McKenney wrote: > > > On Wed, Sep 19, 2007 at 11:16:21AM -0400, Dmitry Torokhov wrote: > > > > On 9/19/07, Peter Zijlstra wrote: > > > > > On Wed, 19 Sep 2007 10:17:25 -0400 "Dmitry Torokhov" > > > > > wrote: > > > > > > > > > > > Hi Peter, > > > > > > > > > > > > On 9/19/07, Peter Zijlstra wrote: > > > > > > > Warn when rcu_dereference() is not used in combination with rcu_read_lock() > > > > > > > > > > > > > > > > > > > According to Paul it is fine to use RCU primitives (when accompanied > > > > > > with proper comments) when the read-size critical section is guarded > > > > > > by spin_lock_irqsave()/spin_lock_irqsrestore() instead of > > > > > > rcu_read_lock()/rcu_read_unlock() and writers synchronize with > > > > > > synchronize_sched(), not synchronize_rcu(). Your patch will trigger > > > > > > warnign on such valid usages. > > > > > > > > > > > > > > > > Sounds fragile to begin with. But you're right in that that is valid > > > > > for Linux as you know it. However in -rt most/all spinlocks are > > > > > converted to sleeping locks. In that case sync_sched() is not enough. > > > > > > > > OK, then it goes beyond RCU... We need to come up with something that > > > > can be used to synchronize with IRQ handlers (quite often in driver > > > > code one needs to be sure that current invocation of IRQ handler > > > > completed before doing something). And once we have it splinlock + RCU > > > > users can just use that method. > > > > > > But Peter's approach would not cause a problem here -- you wouldn't be > > > doing an rcu_dereference from within the IRQ handler in this case, right? > > > > Yes I do. Along with list_for_each_rcu(). > > OK, in that case it does indeed need to be handled. 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? Thanx, Paul