From: Paul E. McKenney Date: Wed, 16 Oct 2013 15:39:10 +0000 (-0700) Subject: rcu: Fix CONFIG_RCU_FANOUT_EXACT for odd fanout/leaf values X-Git-Tag: v3.14-rc1~174^2^2^2~7 X-Git-Url: http://review.tizen.org/git/?a=commitdiff_plain;h=04f34650ca5e8445aae0ab3e0ff6704f141150a8;p=kernel%2Fkernel-generic.git rcu: Fix CONFIG_RCU_FANOUT_EXACT for odd fanout/leaf values 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 --- diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index f8c0299..db1a9fd 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3259,9 +3259,9 @@ 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)