From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752051AbbCGSf5 (ORCPT ); Sat, 7 Mar 2015 13:35:57 -0500 Received: from mx1.redhat.com ([209.132.183.28]:37218 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751065AbbCGSf4 (ORCPT ); Sat, 7 Mar 2015 13:35:56 -0500 Date: Sat, 7 Mar 2015 18:48:21 +0000 From: Alexander Gordeev To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/9] rcu: Panic if RCU tree can not accommodate all CPUs Message-ID: <20150307184821.GA15033@agordeev.usersys.redhat.com> References: <20150307174234.GL5236@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150307174234.GL5236@linux.vnet.ibm.com> User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 07, 2015 at 09:42:34AM -0800, Paul E. McKenney wrote: > 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. The emphasis here the existing check (... n > rcu_capacity[MAX_RCU_LVLS]) (below as [1]) should not cause the fall back to compiled-time values. It either must panic or, as you say - 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]) { [1] > > + 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 > > > -- Regards, Alexander Gordeev agordeev@redhat.com