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Subject[PATCH] llist: Use try_cmpxchg in llist_add_batch and llist_del_first
Date
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.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
---
lib/llist.c | 12 ++++--------
1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/lib/llist.c b/lib/llist.c
index 611ce4881a87..7d78b736e8af 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -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;
}
--
2.35.3
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