Unverified Commit 67d676bb authored by Christian Brauner's avatar Christian Brauner
Browse files

rculist: add list_bidir_{del,prev}_rcu() 

Currently there is no primitive for retrieving the previous list member.
To do this we need a new deletion primitive that doesn't poison the prev
pointer and a corresponding retrieval helper. Note that it is not valid
to ues both list_del_rcu() and list_bidir_del_rcu() on the same list.

Link: https://lore.kernel.org/r/20241213-work-mount-rbtree-lockless-v3-4-6e3cdaf9b280@kernel.org


Reviewed-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Reviewed-by: default avatarJeff Layton <jlayton@kernel.org>
Suggested-by: default avatarPaul E. McKenney <paulmck@kernel.org>
Signed-off-by: default avatarChristian Brauner <brauner@kernel.org>
parent 5dcbd85d

include/linux/rculist.h

+44 −0
Original line number Diff line number Diff line
@@ -30,6 +30,17 @@ static inline void INIT_LIST_HEAD_RCU(struct list_head *list) 
 * way, we must not access it directly

 */

#define list_next_rcu(list)	(*((struct list_head __rcu **)(&(list)->next)))

/*

 * Return the ->prev pointer of a list_head in an rcu safe way. Don't

 * access it directly.

 *

 * Any list traversed with list_bidir_prev_rcu() must never use

 * list_del_rcu().  Doing so will poison the ->prev pointer that

 * list_bidir_prev_rcu() relies on, which will result in segfaults.

 * To prevent these segfaults, use list_bidir_del_rcu() instead

 * of list_del_rcu().

 */

#define list_bidir_prev_rcu(list) (*((struct list_head __rcu **)(&(list)->prev)))



/**

 * list_tail_rcu - returns the prev pointer of the head of the list
@@ -158,6 +169,39 @@ static inline void list_del_rcu(struct list_head *entry) 
	entry->prev = LIST_POISON2;

}



/**

 * list_bidir_del_rcu - deletes entry from list without re-initialization

 * @entry: the element to delete from the list.

 *

 * In contrast to list_del_rcu() doesn't poison the prev pointer thus

 * allowing backwards traversal via list_bidir_prev_rcu().

 *

 * Note: list_empty() on entry does not return true after this because

 * the entry is in a special undefined state that permits RCU-based

 * lockfree reverse traversal. In particular this means that we can not

 * poison the forward and backwards pointers that may still be used for

 * walking the list.

 *

 * The caller must take whatever precautions are necessary (such as

 * holding appropriate locks) to avoid racing with another list-mutation

 * primitive, such as list_bidir_del_rcu() or list_add_rcu(), running on

 * this same list. However, it is perfectly legal to run concurrently

 * with the _rcu list-traversal primitives, such as

 * list_for_each_entry_rcu().

 *

 * Note that list_del_rcu() and list_bidir_del_rcu() must not be used on

 * the same list.

 *

 * Note that the caller is not permitted to immediately free

 * the newly deleted entry.  Instead, either synchronize_rcu()

 * or call_rcu() must be used to defer freeing until an RCU

 * grace period has elapsed.

 */

static inline void list_bidir_del_rcu(struct list_head *entry)

{

	__list_del_entry(entry);

}



/**

 * hlist_del_init_rcu - deletes entry from hash list with re-initialization

 * @n: the element to delete from the hash list.