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@xxxxxxxxx>
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@xxxxxxxxx
Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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@xxxxxxxxx are
llist-use-try_cmpxchg-in-llist_add_batch-and-llist_del_first.patch
