Move some code from hlist_nulls_add_tail_rcu() to the new function hlist_nulls_add_behind_rcu() for callers that already know the last node of an `hlist_nulls` list, to allow constant-time appends.
Signed-off-by: Max Kellermann <[email protected]> --- include/linux/rculist_nulls.h | 36 +++++++++++++++++++++++++++++------ 1 file changed, 30 insertions(+), 6 deletions(-) diff --git a/include/linux/rculist_nulls.h b/include/linux/rculist_nulls.h index a97c3bcb1656..054e477dd093 100644 --- a/include/linux/rculist_nulls.h +++ b/include/linux/rculist_nulls.h @@ -115,6 +115,33 @@ static inline void hlist_nulls_add_head_rcu(struct hlist_nulls_node *n, WRITE_ONCE(first->pprev, &n->next); } +/** + * hlist_nulls_add_behind_rcu - append after the last list entry + * @n: the new element to add to the hash list. + * @last: the last element in the list. + * + * Adds the specified element after @last while permitting racing RCU + * traversals. @last must be the last list entry, with its next + * pointer holding the terminating nulls marker. + * + * The caller must take whatever precautions are necessary + * (such as holding appropriate locks) to avoid racing + * with another list-mutation primitive, such as hlist_nulls_add_head_rcu() + * or hlist_nulls_del_rcu(), running on this same list. + * However, it is perfectly legal to run concurrently with + * the _rcu list-traversal primitives, such as + * hlist_nulls_for_each_entry_rcu(), used to prevent memory-consistency + * problems on Alpha CPUs. Regardless of the type of CPU, the + * list-traversal primitive must be guarded by rcu_read_lock(). + */ +static inline void hlist_nulls_add_behind_rcu(struct hlist_nulls_node *n, + struct hlist_nulls_node *last) +{ + WRITE_ONCE(n->next, last->next); + WRITE_ONCE(n->pprev, &last->next); + rcu_assign_pointer(hlist_nulls_next_rcu(last), n); +} + /** * hlist_nulls_add_tail_rcu * @n: the element to add to the hash list. @@ -143,13 +170,10 @@ static inline void hlist_nulls_add_tail_rcu(struct hlist_nulls_node *n, for (i = h->first; !is_a_nulls(i); i = i->next) last = i; - if (last) { - WRITE_ONCE(n->next, last->next); - WRITE_ONCE(n->pprev, &last->next); - rcu_assign_pointer(hlist_nulls_next_rcu(last), n); - } else { + if (last) + hlist_nulls_add_behind_rcu(n, last); + else hlist_nulls_add_head_rcu(n, h); - } } /* after that hlist_nulls_del will work */ -- 2.47.3
