Re: [PATCH 1/1] rculist: Made list_first_entry_rcu usable

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

On Mon, Apr 02, 2012 at 09:42:34PM -0400, Michel Machado wrote:
> 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
> snippet:
> 
> if (!list_empty_rcu(mylist)) {
> 	struct foo *bar = list_first_entry_rcu(mylist, struct foo,
> 		list_member);
> 	do_something(bar);
> }
> 
> 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.

Hello, Michel,

Interesting point!

Are you intending to use list_first_entry_rcu()?  If not, perhaps the
best thing to do is to remove it.

							Thanx, Paul

> Signed-off-by: Michel Machado <michel@digirati.com.br>
> CC: Dipankar Sarma <dipankar@in.ibm.com>
> CC: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> ---
> 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
> list-mutation
>   * primitives such as list_add_rcu() as long as it's guarded by
> rcu_read_lock().
>   */
>  #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
> 
>