From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755585Ab0EEXGF (ORCPT ); Wed, 5 May 2010 19:06:05 -0400 Received: from e9.ny.us.ibm.com ([32.97.182.139]:57135 "EHLO e9.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754856Ab0EEXGB (ORCPT ); Wed, 5 May 2010 19:06:01 -0400 Date: Wed, 5 May 2010 16:05:57 -0700 From: "Paul E. McKenney" To: Mathieu Desnoyers Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com Subject: Re: [PATCH tip/core/rcu 01/48] rcu: optionally leave lockdep enabled after RCU lockdep splat Message-ID: <20100505230557.GD2439@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100504201934.GA19234@linux.vnet.ibm.com> <1273004398-19760-1-git-send-email-paulmck@linux.vnet.ibm.com> <20100505224641.GA15359@Krystal> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100505224641.GA15359@Krystal> User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, May 05, 2010 at 06:46:41PM -0400, Mathieu Desnoyers wrote: > * Paul E. McKenney (paulmck@linux.vnet.ibm.com) wrote: > > From: Lai Jiangshan > > > > There is no need to disable lockdep after an RCU lockdep splat, > > so remove the debug_lockdeps_off() from lockdep_rcu_dereference(). > > To avoid repeated lockdep splats, use a static variable in the inlined > > rcu_dereference_check() and rcu_dereference_protected() macros so that > > a given instance splats only once, but so that multiple instances can > > be detected per boot. > > > > This is controlled by a new config variable CONFIG_PROVE_RCU_REPEATEDLY, > > which is disabled by default. This provides the normal lockdep behavior > > by default, but permits people who want to find multiple RCU-lockdep > > splats per boot to easily do so. > > I'll play the devil's advocate here. (just because that's so much fun) > ;-) > > If we look at: > > include/linux/debug_locks.h: > > static inline int __debug_locks_off(void) > { > return xchg(&debug_locks, 0); > } > > We see that all code following a call to "debug_locks_off()" can assume > that it cannot possibly run concurrently with other code following > "debug_locks_off()". Now, I'm not saying that the code we currently have > will necessarily break, but I think it is worth asking if there is any > assumption in lockdep (or RCU lockdep more specifically) about mutual > exclusion after debug_locks_off() ? > > Because if there is, then this patch is definitely breaking something by > not protecting lockdep against multiple concurrent executions. So what in lockdep_rcu_dereference() needs to be protected from concurrent execution? All that happens beyond that point is a bunch of printk()s, printing the locks held by this task, and dumping this task's stack. Thanx, Paul