linux-kernel - [PATCH] llist: Use try_cmpxchg in llist_add_batch and llist_del

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Date:   Tue, 12 Jul 2022 16:49:17 +0200
From:   Uros Bizjak <ubizjak@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     Uros Bizjak <ubizjak@...il.com>
Subject: [PATCH] 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.

Signed-off-by: Uros Bizjak <ubizjak@...il.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