From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754241Ab3KPAZh (ORCPT ); Fri, 15 Nov 2013 19:25:37 -0500 Received: from e37.co.us.ibm.com ([32.97.110.158]:57552 "EHLO e37.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753810Ab3KPAXs (ORCPT ); Fri, 15 Nov 2013 19:23:48 -0500 From: "Paul E. McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, laijs@cn.fujitsu.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, niv@us.ibm.com, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, darren@dvhart.com, fweisbec@gmail.com, sbw@mit.edu, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 08/16] rcu: Fix CONFIG_RCU_FANOUT_EXACT for odd fanout/leaf values Date: Fri, 15 Nov 2013 16:23:30 -0800 Message-Id: <1384561418-30575-8-git-send-email-paulmck@linux.vnet.ibm.com> X-Mailer: git-send-email 1.8.1.5 In-Reply-To: <1384561418-30575-1-git-send-email-paulmck@linux.vnet.ibm.com> References: <20131116002316.GA29049@linux.vnet.ibm.com> <1384561418-30575-1-git-send-email-paulmck@linux.vnet.ibm.com> X-TM-AS-MML: disable X-Content-Scanned: Fidelis XPS MAILER x-cbid: 13111600-7164-0000-0000-0000035AF092 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Paul E. McKenney" Each element of the rcu_state structure's ->levelspread[] array is intended to contain the per-level fanout, where the zero-th element corresponds to the root of the rcu_node tree, and the last element corresponds to the leaves. In the CONFIG_RCU_FANOUT_EXACT case, this means that the last element should be filled in from CONFIG_RCU_FANOUT_LEAF (or from the rcu_fanout_leaf boot parameter, if provided) and that the remaining elements should be filled in from CONFIG_RCU_FANOUT. Unfortunately, the current code in rcu_init_levelspread() takes the opposite approach, placing CONFIG_RCU_FANOUT_LEAF in the zero-th element and CONFIG_RCU_FANOUT in the remaining elements. For typical power-of-two values, this generates odd but functional rcu_node trees. However, other values, for example CONFIG_RCU_FANOUT=3 and CONFIG_RCU_FANOUT_LEAF=2, generate trees that can leave some CPUs out of the grace-period computation, resulting in too-short grace periods and therefore a broken RCU implementation. This commit therefore fixes rcu_init_levelspread() to set the last ->levelspread[] array element from CONFIG_RCU_FANOUT_LEAF and the remaining elements from CONFIG_RCU_FANOUT, thus generating the intended rcu_node trees. Signed-off-by: Paul E. McKenney --- kernel/rcu/tree.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index e0a58eca0092..13d1a1a0d60a 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3246,9 +3246,9 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp) { int i; - for (i = rcu_num_lvls - 1; i > 0; i--) + rsp->levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf; + for (i = rcu_num_lvls - 2; i >= 0; i--) rsp->levelspread[i] = CONFIG_RCU_FANOUT; - rsp->levelspread[0] = rcu_fanout_leaf; } #else /* #ifdef CONFIG_RCU_FANOUT_EXACT */ static void __init rcu_init_levelspread(struct rcu_state *rsp) -- 1.8.1.5