From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751560AbbCGRmm (ORCPT ); Sat, 7 Mar 2015 12:42:42 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:50786 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751079AbbCGRml (ORCPT ); Sat, 7 Mar 2015 12:42:41 -0500 Date: Sat, 7 Mar 2015 09:42:34 -0800 From: "Paul E. McKenney" To: Alexander Gordeev Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs Message-ID: <20150307174234.GL5236@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15030717-0025-0000-0000-00000919D911 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 07, 2015 at 06:03:36PM +0100, Alexander Gordeev wrote: > Currently a condition when RCU tree is unable to accommodate > the configured number of CPUs is not permitted and causes > a fall back to compile-time values. However, the code has no > means to exceed the RCU tree capacity neither at compile-time > nor in run-time. Therefore, if the condition is met in run- > time then it indicates a serios problem elsewhere and should > be handled with a panic. > > Cc: "Paul E. McKenney" > Signed-off-by: Alexander Gordeev The place to put a check like this is in the code that calculates nr_cpu_ids. And at least some (perhaps all) are set up so that nr_cpu_ids cannot exceed NR_CPUS, which would render this check redundant. So I have to say "no" to this one. Thanx, Paul > --- > kernel/rcu/tree.c | 15 +++++++++------ > 1 file changed, 9 insertions(+), 6 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 48d640c..7588c7f 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3889,16 +3889,19 @@ static void __init rcu_init_geometry(void) > rcu_capacity[i] = rcu_capacity[i - 1] * CONFIG_RCU_FANOUT; > > /* > + * The tree must be able to accommodate the configured number of CPUs. > + * If this limit is exceeded than we have a serious problem elsewhere. > + * > * The boot-time rcu_fanout_leaf parameter is only permitted > * to increase the leaf-level fanout, not decrease it. Of course, > * the leaf-level fanout cannot exceed the number of bits in > - * the rcu_node masks. Finally, the tree must be able to accommodate > - * the configured number of CPUs. Complain and fall back to the > - * compile-time values if these limits are exceeded. > + * the rcu_node masks. Complain and fall back to the compile- > + * time values if these limits are exceeded. > */ > - if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF || > - rcu_fanout_leaf > sizeof(unsigned long) * 8 || > - n > rcu_capacity[MAX_RCU_LVLS]) { > + if (n > rcu_capacity[MAX_RCU_LVLS]) > + panic("rcu_init_geometry: rcu_capacity[] is too small"); > + else if (rcu_fanout_leaf < CONFIG_RCU_FANOUT_LEAF || > + rcu_fanout_leaf > sizeof(unsigned long) * 8) { > WARN_ON(1); > return; > } > -- > 1.8.3.1 >