Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752153Ab1BWOhv (ORCPT ); Wed, 23 Feb 2011 09:37:51 -0500 Received: from blu0-omc1-s31.blu0.hotmail.com ([65.55.116.42]:56814 "EHLO blu0-omc1-s31.blu0.hotmail.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751658Ab1BWOhu (ORCPT ); Wed, 23 Feb 2011 09:37:50 -0500 X-Originating-IP: [174.91.193.52] X-Originating-Email: [pdumas9@sympatico.ca] Message-ID: Date: Wed, 23 Feb 2011 09:37:48 -0500 From: Mathieu Desnoyers To: Steven Rostedt CC: "Paul E. McKenney" , linux-kernel@vger.kernel.org, mingo@elte.hu, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, Valdis.Kletnieks@vt.edu, dhowells@redhat.com, eric.dumazet@gmail.com, darren@dvhart.com Subject: Re: [PATCH RFC tip/core/rcu 05/11] rcu: add comment saying why DEBUG_OBJECTS_RCU_HEAD depends on PREEMPT. References: <20110223013917.GA20996@linux.vnet.ibm.com> <1298425183-21265-5-git-send-email-paulmck@linux.vnet.ibm.com> <1298431409.7666.76.camel@gandalf.stny.rr.com> <1298470303.7666.80.camel@gandalf.stny.rr.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Content-Disposition: inline In-Reply-To: <1298470303.7666.80.camel@gandalf.stny.rr.com> X-Editor: vi X-Info: http://krystal.dyndns.org:8080 X-Operating-System: Linux/2.6.27.31-grsec (i686) X-Uptime: 09:37:13 up 322 days, 26 min, 6 users, load average: 1.65, 1.53, 1.50 User-Agent: Mutt/1.5.18 (2008-05-17) X-OriginalArrivalTime: 23 Feb 2011 14:37:49.0809 (UTC) FILETIME=[3EC08E10:01CBD367] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3276 Lines: 88 * Steven Rostedt (rostedt@goodmis.org) wrote: > On Wed, 2011-02-23 at 08:59 -0500, Mathieu Desnoyers wrote: > > * Steven Rostedt (rostedt@goodmis.org) wrote: > > > On Tue, 2011-02-22 at 17:39 -0800, Paul E. McKenney wrote: > > > > The build will break if you change the Kconfig to allow > > > > DEBUG_OBJECTS_RCU_HEAD and !PREEMPT, so document the reasoning > > > > near where the breakage would occur. > > > > > > > > Signed-off-by: Paul E. McKenney > > > > --- > > > > kernel/rcupdate.c | 5 +++++ > > > > 1 files changed, 5 insertions(+), 0 deletions(-) > > > > > > > > diff --git a/kernel/rcupdate.c b/kernel/rcupdate.c > > > > index afd21d1..f3240e9 100644 > > > > --- a/kernel/rcupdate.c > > > > +++ b/kernel/rcupdate.c > > > > @@ -214,6 +214,11 @@ static int rcuhead_fixup_free(void *addr, enum debug_obj_state state) > > > > * Ensure that queued callbacks are all executed. > > > > * If we detect that we are nested in a RCU read-side critical > > > > * section, we should simply fail, otherwise we would deadlock. > > > > + * Note that the machinery to reliably determine whether > > > > + * or not we are in an RCU read-side critical section > > > > + * exists only in the preemptible RCU implementations > > > > + * (TINY_PREEMPT_RCU and TREE_PREEMPT_RCU), which is why > > > > + * DEBUG_OBJECTS_RCU_HEAD is disallowed if !PREEMPT. > > > > */ > > > > > > Shouldn't this comment also be in the kconfig where > > > DEBUG_OBJECTS_RCU_HEAD is defined? > > > > hrm, but this is a "rcuhead_fixup_init" : it does not need to always > > Looks like it was in rcuhead_fixup_free to me. OK. > > > succeed. It's just that when it is safe to recover from an error > > situation, it does it. We could do: > > > > #ifdef CONFIG_PREEMPT > > /* > > * Ensure that queued callbacks are all executed. > > * If we detect that we are nested in a RCU read-side > > * critical > > * section, we should simply fail, otherwise we would > > * deadlock. > > */ > > if (rcu_preempt_depth() != 0 || preempt_count() != 0 || > > irqs_disabled()) { > > WARN_ON(1); > > return 0; > > } > > rcu_barrier(); > > rcu_barrier_sched(); > > rcu_barrier_bh(); > > debug_object_init(head, &rcuhead_debug_descr); > > return 1; > > #else > > return 0; > > #endif > > > > instead, no ? > > The point is that this entire block of code is wrapped in #ifdef > CONFIG_DEBUG_OBJECTS_RCU_HEAD, and that config depends on PREEMPT. Thus > you will never have a case where #ifdef CONFIG_PREEMPT is false. Why does CONFIG_DEBUG_OBJECTS_RCU_HEAD depend on PREEMPT again ? Mathieu > > -- Steve > > -- Mathieu Desnoyers Operating System Efficiency R&D Consultant EfficiOS Inc. http://www.efficios.com -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/