Re: [TOMOYO #14 (mmotm 2008-12-30-16-05) 02/10] Singly linked listimplementation.

[James Morris]

On Thu, 1 Jan 2009, Tetsuo Handa wrote:

> This patch introduces new list structure named "list1".
> 
> TOMOYO wants to use "write once read many" list.
> Since only two operations
> 
>  (1) Append an entry to the tail of the list.
>  (2) Read all entries starting from the head of the list.
> 
> are needed for that purpose, this list doesn't have pointer to previous
> element.
> 
> While "hlist" is defined as
> 
>  struct hlist_head {
>  	struct hlist_node *first;
>  };
> 
>  struct hlist_node {
>  	struct hlist_node *next, **pprev;
>  };
> 
> I don't use "hlist_node" because I don't need pointer to previous element.
> 
> By ommiting pointer to previous element, the reader becomes read lock free,
> which is good thing for implementing "write once read many" list.

This has a technical ack from Paul, but what about Linus' long-standing 
objection to singly-linked lists in the kernel?  I'm sure this has been 
discussed re. your patches, but I can't find a reference.

Also, should this be folded into list.h, and is "list1" an appropriate 
name?  ("slist" might be better).

 > 
> Signed-off-by: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>  include/linux/list1.h |   81 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 81 insertions(+)
> 
> --- /dev/null
> +++ linux-2.6.28-mm1/include/linux/list1.h
> @@ -0,0 +1,81 @@
> +#ifndef _LINUX_LIST1_H
> +#define _LINUX_LIST1_H
> +
> +#include <linux/list.h>
> +#include <linux/rcupdate.h>
> +
> +/*
> + * Singly linked list implementation.
> + *
> + * This list supports only two operations.
> + * (1) Append an entry to the tail of the list.
> + * (2) Read all entries starting from the head of the list.
> + *
> + * This list is designed for holding "write once, read many" entries.
> + * This list requires no locks for read operation.
> + * This list doesn't support "remove an entry from the list" operation.
> + */
> +
> +/* To reduce memory usage, this list doesn't use "->prev" pointer. */
> +struct list1_head {
> +	struct list1_head *next;
> +};
> +
> +#define LIST1_HEAD_INIT(name) { &(name) }
> +#define LIST1_HEAD(name) struct list1_head name = LIST1_HEAD_INIT(name)
> +
> +static inline void INIT_LIST1_HEAD(struct list1_head *list)
> +{
> +	list->next = list;
> +}
> +
> +/* Reuse list_entry because it doesn't use "->prev" pointer. */
> +#define list1_entry list_entry
> +
> +/* Reuse list_for_each_rcu because it doesn't use "->prev" pointer. */
> +#define list1_for_each list_for_each_rcu
> +/* Reuse list_for_each_entry_rcu because it doesn't use "->prev" pointer. */
> +#define list1_for_each_entry list_for_each_entry_rcu
> +
> +/**
> + * list1_for_each_cookie - iterate over a list with cookie.
> + * @pos:        the &struct list1_head to use as a loop cursor.
> + * @cookie:     the &struct list1_head to use as a cookie.
> + * @head:       the head for your list.
> + *
> + * Same with list_for_each_rcu() except that this primitive uses @cookie
> + * so that we can continue iteration.
> + * @cookie must be NULL when iteration starts, and @cookie will become
> + * NULL when iteration finishes.
> + *
> + * Since list elements are never removed, we don't need to get a lock
> + * or a reference count.
> + */
> +#define list1_for_each_cookie(pos, cookie, head)                      \
> +	for (({ if (!cookie)                                          \
> +				     cookie = head; }),               \
> +	     pos = rcu_dereference((cookie)->next);                   \
> +	     prefetch(pos->next), pos != (head) || ((cookie) = NULL); \
> +	     (cookie) = pos, pos = rcu_dereference(pos->next))
> +
> +/**
> + * list1_add_tail - add a new entry to list1 list.
> + * @new: new entry to be added.
> + * @head: list head to add it before.
> + *
> + * Same with list_add_tail_rcu() without "->prev" pointer.
> + *
> + * Caller must hold a lock for protecting @head.
> + */
> +static inline void list1_add_tail(struct list1_head *new,
> +				  struct list1_head *head)
> +{
> +	struct list1_head *prev = head;
> +
> +	new->next = head;
> +	while (prev->next != head)
> +		prev = prev->next;
> +	rcu_assign_pointer(prev->next, new);
> +}
> +
> +#endif
> 
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-- 
James Morris
<jmorris@xxxxxxxxx>
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