From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754614Ab0AECTY (ORCPT ); Mon, 4 Jan 2010 21:19:24 -0500 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1754431Ab0AECTX (ORCPT ); Mon, 4 Jan 2010 21:19:23 -0500 Received: from e3.ny.us.ibm.com ([32.97.182.143]:60564 "EHLO e3.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754256Ab0AECTV (ORCPT ); Mon, 4 Jan 2010 21:19:21 -0500 Date: Mon, 4 Jan 2010 18:19:19 -0800 From: "Paul E. McKenney" To: Josh Triplett Cc: linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@polymtl.ca, dvhltc@us.ibm.com, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com Subject: Re: [PATCH tip/core/rcu 2/3] rcu: add debug check for too many rcu_read_unlock() Message-ID: <20100105021919.GN6748@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100105000337.GA23307@linux.vnet.ibm.com> <20100105020307.GB11286@feather> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20100105020307.GB11286@feather> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jan 04, 2010 at 06:03:08PM -0800, Josh Triplett wrote: > On Mon, Jan 04, 2010 at 04:04:01PM -0800, Paul E. McKenney wrote: > > From: Paul E. McKenney > > > > TREE_PREEMPT_RCU maintains an rcu_read_lock_nesting counter in the > > task structure, which happens to be a signed int. So this patch adds a > > check for this counter being negative at the end of __rcu_read_unlock(). > > This check is under CONFIG_PROVE_LOCKING, so can be thought of as being > > part of lockdep. > > > > Signed-off-by: Paul E. McKenney > > --- > > kernel/rcutree_plugin.h | 3 +++ > > 1 files changed, 3 insertions(+), 0 deletions(-) > > > > diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h > > index f11ebd4..e77cdf3 100644 > > --- a/kernel/rcutree_plugin.h > > +++ b/kernel/rcutree_plugin.h > > @@ -304,6 +304,9 @@ void __rcu_read_unlock(void) > > if (--ACCESS_ONCE(t->rcu_read_lock_nesting) == 0 && > > unlikely(ACCESS_ONCE(t->rcu_read_unlock_special))) > > rcu_read_unlock_special(t); > > +#ifdef CONFIG_PROVE_LOCKING > > + WARN_ON_ONCE(ACCESS_ONCE(t->rcu_read_lock_nesting) < 0); > > +#endif /* #ifdef CONFIG_PROVE_LOCKING */ > > } > > EXPORT_SYMBOL_GPL(__rcu_read_unlock); > > Given that you *already* need to access t->rcu_read_lock_nesting here, > why not just do the test all the time? Ideally you could access > t->rcu_read_lock_nesting once, decrement it, and test for both 0 and > negative. Because I was paranoid about the extra branch. Perhaps needlessly paranoid, but this is rcu_read_unlock() we are talking about here. ;-) You seem to be suggesting making the first test be "<=", then sorting things out later, but given that both the equals-zero and the greater-than-zero cases are quite common, I couldn't figure out how to avoid the extra test and branch in the common case. Hence the #ifdef. Thanx, Paul