From b835db1f9cadaf008750a32664e35a207782c95e Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 8 Sep 2009 15:54:37 -0700 Subject: [PATCH] rcu: Initialize multi-level RCU grace periods holding locks Prior implementations initialized the root and any internal nodes without holding locks, then initialized the leaves holding locks. This is a false economy, as the leaf nodes will usually greatly outnumber the root and internal nodes. Acquiring locks on all nodes is conceptually much simpler as well. Signed-off-by: Paul E. McKenney Acked-by: Steven Rostedt Cc: laijs@cn.fujitsu.com Cc: dipankar@in.ibm.com Cc: akpm@linux-foundation.org Cc: mathieu.desnoyers@polymtl.ca Cc: josht@linux.vnet.ibm.com Cc: dvhltc@us.ibm.com Cc: niv@us.ibm.com Cc: peterz@infradead.org LKML-Reference: <12524504773190-git-send-email-> Signed-off-by: Ingo Molnar --- kernel/rcutree.c | 41 ++++++++++++----------------------------- 1 file changed, 12 insertions(+), 29 deletions(-) diff --git a/kernel/rcutree.c b/kernel/rcutree.c index c634a92d1217..da301e2fd84f 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -645,41 +645,24 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags) spin_lock(&rsp->onofflock); /* irqs already disabled. */ /* - * Set the quiescent-state-needed bits in all the non-leaf RCU - * nodes for all currently online CPUs. This operation relies - * on the layout of the hierarchy within the rsp->node[] array. - * Note that other CPUs will access only the leaves of the - * hierarchy, which still indicate that no grace period is in - * progress. In addition, we have excluded CPU-hotplug operations. - * - * We therefore do not need to hold any locks. Any required - * memory barriers will be supplied by the locks guarding the - * leaf rcu_nodes in the hierarchy. - */ - - rnp_end = rsp->level[NUM_RCU_LVLS - 1]; - for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) { - rnp_cur->qsmask = rnp_cur->qsmaskinit; - rnp->gpnum = rsp->gpnum; - } - - /* - * Now set up the leaf nodes. Here we must be careful. First, - * we need to hold the lock in order to exclude other CPUs, which - * might be contending for the leaf nodes' locks. Second, as - * soon as we initialize a given leaf node, its CPUs might run - * up the rest of the hierarchy. We must therefore acquire locks - * for each node that we touch during this stage. (But we still - * are excluding CPU-hotplug operations.) + * Set the quiescent-state-needed bits in all the rcu_node + * structures for all currently online CPUs in breadth-first + * order, starting from the root rcu_node structure. This + * operation relies on the layout of the hierarchy within the + * rsp->node[] array. Note that other CPUs will access only + * the leaves of the hierarchy, which still indicate that no + * grace period is in progress, at least until the corresponding + * leaf node has been initialized. In addition, we have excluded + * CPU-hotplug operations. * * Note that the grace period cannot complete until we finish * the initialization process, as there will be at least one * qsmask bit set in the root node until that time, namely the - * one corresponding to this CPU. + * one corresponding to this CPU, due to the fact that we have + * irqs disabled. */ rnp_end = &rsp->node[NUM_RCU_NODES]; - rnp_cur = rsp->level[NUM_RCU_LVLS - 1]; - for (; rnp_cur < rnp_end; rnp_cur++) { + for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) { spin_lock(&rnp_cur->lock); /* irqs already disabled. */ rnp_cur->qsmask = rnp_cur->qsmaskinit; rnp->gpnum = rsp->gpnum; -- 2.30.2