Commit 4f1d2a03 authored by Uros Bizjak's avatar Uros Bizjak Committed by Andrew Morton
Browse files

llist: use try_cmpxchg in llist_add_batch and llist_del_first 

Use try_cmpxchg instead of cmpxchg (*ptr, old, new) == old in
llist_add_batch and llist_del_first.  x86 CMPXCHG instruction returns
success in ZF flag, so this change saves a compare after cmpxchg.

Also, try_cmpxchg implicitly assigns old *ptr value to "old" when cmpxchg
fails, enabling further code simplifications.

No functional change intended.

Link: https://lkml.kernel.org/r/20220712144917.4497-1-ubizjak@gmail.com


Signed-off-by: default avatarUros Bizjak <ubizjak@gmail.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 05c62574

lib/llist.c

+4 −8
Original line number Diff line number Diff line
@@ -30,7 +30,7 @@ bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last, 


	do {

		new_last->next = first = READ_ONCE(head->first);

	} while (cmpxchg(&head->first, first, new_first) != first);

	} while (!try_cmpxchg(&head->first, &first, new_first));



	return !first;

}
@@ -52,18 +52,14 @@ EXPORT_SYMBOL_GPL(llist_add_batch); 
 */

struct llist_node *llist_del_first(struct llist_head *head)

{

	struct llist_node *entry, *old_entry, *next;

	struct llist_node *entry, *next;



	entry = smp_load_acquire(&head->first);

	for (;;) {

	do {

		if (entry == NULL)

			return NULL;

		old_entry = entry;

		next = READ_ONCE(entry->next);

		entry = cmpxchg(&head->first, old_entry, next);

		if (entry == old_entry)

			break;

	}

	} while (!try_cmpxchg(&head->first, &entry, next));



	return entry;

}