+ llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch added to mm-nonmm-unstable branch

[Andrew Morton <akpm@linux-foundation.org>]

The patch titled
     Subject: llist: use try_cmpxchg in llist_add_batch and llist_del_first
has been added to the -mm mm-nonmm-unstable branch.  Its filename is
     llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch

This patch will shortly appear at
     https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch

This patch will later appear in the mm-nonmm-unstable branch at
    git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

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------------------------------------------------------
From: Uros Bizjak <ubizjak@gmail.com>
Subject: llist: use try_cmpxchg in llist_add_batch and llist_del_first
Date: Tue, 12 Jul 2022 16:49:17 +0200

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: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---

 lib/llist.c |   12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

--- a/lib/llist.c~llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first
+++ a/lib/llist.c
@@ -30,7 +30,7 @@ bool llist_add_batch(struct llist_node *
 
 	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;
 }
_

Patches currently in -mm which might be from ubizjak@gmail.com are

llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch