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    Subject[PATCH 1/1] rculist: Made list_first_entry_rcu usable
    The macro list_first_entry_rcu assumed that the passed list is not empty
    as its counterpart list_first_entry does. However, one can test that a
    list is not empty with list_empty before calling list_first_entry,
    whereas neither exists list_empty_rcu, nor is advisable to add it as the
    example below shows.

    Assuming that list_empty_rcu is available, one could write the following

    if (!list_empty_rcu(mylist)) {
    struct foo *bar = list_first_entry_rcu(mylist, struct foo,

    The problem with this snippet is the following racing condition: the
    list may not be empty when list_empty_rcu checks it, but it may be when
    list_first_entry_rcu rereads the ->next pointer.

    This patch cannot break any upstream code because list_first_entry_rcu
    is not being used anywhere in the kernel (tested with grep(1)), and
    external code that uses it is probably broken already.

    Signed-off-by: Michel Machado <>
    CC: Dipankar Sarma <>
    CC: "Paul E. McKenney" <>
    Please CC my e-mail address while replying this message because I don't
    subscribe this mailing list due to its high volume; thanks.

    diff --git a/include/linux/rculist.h b/include/linux/rculist.h
    index d079290..866d3ec 100644
    --- a/include/linux/rculist.h
    +++ b/include/linux/rculist.h
    @@ -233,13 +233,16 @@ static inline void list_splice_init_rcu(struct
    list_head *list,
    * @type: the type of the struct this is embedded in.
    * @member: the name of the list_struct within the struct.
    - * Note, that list is expected to be not empty.
    + * Note that if the list is empty, it returns NULL.
    * This primitive may safely run concurrently with the _rcu
    * primitives such as list_add_rcu() as long as it's guarded by
    #define list_first_entry_rcu(ptr, type, member) \
    - list_entry_rcu((ptr)->next, type, member)
    + ({struct list_head *__ptr = ptr; \
    + struct list_head __rcu *__next = list_next_rcu(__ptr); \
    + likely(__ptr != __next) ? container_of(__next, type, member) : NULL;
    + })

    * list_for_each_entry_rcu - iterate over rcu list of given type

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      Last update: 2012-04-03 03:45    [W:0.039 / U:47.808 seconds]
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