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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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