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Subject[PATCH] add an RCU version of list splicing
This patch is in support of the IPMI driver.  I have tested this
with the IPMI driver changes coming in the next patch.

Add a list_splice_init_rcu() function to splice an RCU-protected list
into another list. This takes the sync function as an argument, so
one would do something like:

INIT_LIST_HEAD(&list);
list_splice_init_rcu(&source, &dest, synchronize_rcu);

The idea being to keep the RCU API proliferation down to a dull roar.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
---
list.h | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 58 insertions(+)
Index: linux-2.6.19/include/linux/list.h
===================================================================
--- linux-2.6.19.orig/include/linux/list.h 2006-11-29 15:57:37.000000000 -0600
+++ linux-2.6.19/include/linux/list.h 2006-12-30 12:47:07.000000000 -0600
@@ -360,6 +360,63 @@
}

/**
+ * list_splice_init_rcu - splice an RCU-protected list into an existing list.
+ * @list: the RCU-protected list to splice
+ * @head: the place in the list to splice the first list into
+ * @sync: function to sync: synchronize_rcu(), synchronize_sched(), ...
+ *
+ * @head can be RCU-read traversed concurrently with this function.
+ *
+ * Note that this function blocks.
+ *
+ * Important note: the caller must take whatever action is necessary to
+ * prevent any other updates to @head. In principle, it is possible
+ * to modify the list as soon as sync() begins execution.
+ * If this sort of thing becomes necessary, an alternative version
+ * based on call_rcu() could be created. But only if -really-
+ * needed -- there is no shortage of RCU API members.
+ */
+static inline void list_splice_init_rcu(struct list_head *list,
+ struct list_head *head,
+ void (*sync)(void))
+{
+ struct list_head *first = list->next;
+ struct list_head *last = list->prev;
+ struct list_head *at = head->next;
+
+ might_sleep();
+ if (list_empty(head))
+ return;
+
+ /* "first" and "last" tracking list, so initialize it. */
+
+ INIT_LIST_HEAD(list);
+
+ /*
+ * At this point, the list body still points to the source list.
+ * Wait for any readers to finish using the list before splicing
+ * the list body into the new list. Any new readers will see
+ * an empty list.
+ */
+
+ sync();
+
+ /*
+ * Readers are finished with the source list, so perform splice.
+ * The order is important if the new list is global and accessible
+ * to concurrent RCU readers. Note that RCU readers are not
+ * permitted to traverse the prev pointers without excluding
+ * this function.
+ */
+
+ last->next = at;
+ smp_wmb();
+ head->next = first;
+ first->prev = head;
+ at->prev = last;
+}
+
+/**
* list_entry - get the struct for this entry
* @ptr: the &struct list_head pointer.
* @type: the type of the struct this is embedded in.
-
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