rcu: Replace list_first_entry_rcu() with list_first_or_null_rcu()

	})

/**

 * list_first_entry_rcu - get the first element from a list

 * Where are list_empty_rcu() and list_first_entry_rcu()?

 *

 * Implementing those functions following their counterparts list_empty() and

 * list_first_entry() is not advisable because they lead to subtle race

 * conditions as the following snippet shows:

 *

 * if (!list_empty_rcu(mylist)) {

 *	struct foo *bar = list_first_entry_rcu(mylist, struct foo, list_member);

 *	do_something(bar);

 * }

 *

 * 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.

 *

 * Rereading the ->next pointer is not a problem for list_empty() and

 * list_first_entry() because they would be protected by a lock that blocks

 * writers.

 *

 * See list_first_or_null_rcu for an alternative.

 */

/**

 * list_first_or_null_rcu - get the first element from a list

 * @ptr:        the list head to take the element from.

 * @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)

#define list_first_or_null_rcu(ptr, 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