[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.

[akpm@osdl.org: build fix]
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Corey Minyard <minyard@acm.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 56 insertions, 0 deletions

diff --git a/include/linux/list.h b/include/linux/list.h
index 611059d633f4..cdc96559e5ae 100644
--- a/include/linux/list.h
+++ b/include/linux/list.h
@@ -360,6 +360,62 @@ static inline void list_splice_init(struct list_head *list,
 }
 
 /**
+ * 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;
+
+        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.