From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932243AbbFBMhx (ORCPT ); Tue, 2 Jun 2015 08:37:53 -0400 Received: from e34.co.us.ibm.com ([32.97.110.152]:44608 "EHLO e34.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752948AbbFBMhq (ORCPT ); Tue, 2 Jun 2015 08:37:46 -0400 Date: Tue, 2 Jun 2015 05:37:40 -0700 From: "Paul E. McKenney" To: Alexander Gordeev Cc: linux-kernel@vger.kernel.org, Steven Rostedt Subject: Re: [PATCH v2 1/9] rcu: Panic if RCU tree can not accommodate all CPUs Message-ID: <20150602123740.GF5989@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <1e5e5cf188c90193dc176caa5b6270606ae7af5b.1432889921.git.agordeev@redhat.com> <20150601183720.GY5989@linux.vnet.ibm.com> <20150602064527.GA3679@agordeev.usersys.redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150602064527.GA3679@agordeev.usersys.redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 15060212-0017-0000-0000-00000B50B3ED Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 02, 2015 at 07:45:27AM +0100, Alexander Gordeev wrote: > On Mon, Jun 01, 2015 at 11:37:21AM -0700, Paul E. McKenney wrote: > > On Fri, May 29, 2015 at 11:53:37AM +0200, 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" > > > Cc: Steven Rostedt > > > Signed-off-by: Alexander Gordeev > > > --- > > > 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 2fce662..66a4230 100644 > > > --- a/kernel/rcu/tree.c > > > +++ b/kernel/rcu/tree.c > > > @@ -4117,16 +4117,19 @@ static void __init rcu_init_geometry(void) > > > rcu_capacity[i] = rcu_capacity[i - 1] * 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 < 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"); > > > > The way this is set up, if the boot parameter (illegally) sets > > rcu_fanout_lead smaller than RCU_FANOUT_LEAF, we might panic. It would > > be far better to first check for rcu_fanout_leaf being out of bounds, > > and only then have the possibility of panic(). That way, a typo in > > the rcu_fanout_leaf boot paremeter is ignored, but with a splat. > > > > Or am I missing something here? > > I think you are quite right. But the bounds check is misplaced then. > I would say, it should be placed before rcu_capacity[] seed, as it > only deals with constants and has nothing with rcu_capacity[]. That makes sense as well. > I will send the updated version. Very good, looking forward to it! By the way, on the specific configurations that I test, the patch generates the same topology as previously, which is reassuring. An exhaustive test is needed, of course. Thanx, Paul > > > + else if (rcu_fanout_leaf < RCU_FANOUT_LEAF || > > > + rcu_fanout_leaf > sizeof(unsigned long) * 8) { > > > WARN_ON(1); > > > return; > > > } > > > -- > > > 1.8.3.1 > > > > > > > -- > > 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/ > > -- > Regards, > Alexander Gordeev > agordeev@redhat.com >