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Subject[tip:core/urgent] rcu: Initialize multi-level RCU grace periods holding locks
Commit-ID:  b835db1f9cadaf008750a32664e35a207782c95e
Gitweb: http://git.kernel.org/tip/b835db1f9cadaf008750a32664e35a207782c95e
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
AuthorDate: Tue, 8 Sep 2009 15:54:37 -0700
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Fri, 18 Sep 2009 00:05:14 +0200

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 <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
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 <mingo@elte.hu>


---
kernel/rcutree.c | 41 ++++++++++++-----------------------------
1 files changed, 12 insertions(+), 29 deletions(-)
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index c634a92..da301e2 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;

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