Commit fb4214db authored by Oleg Nesterov's avatar Oleg Nesterov Committed by Al Viro
Browse files

llist: fix/simplify llist_add() and llist_add_batch()



1. This is mostly theoretical, but llist_add*() need ACCESS_ONCE().

   Otherwise it is not guaranteed that the first cmpxchg() uses the
   same value for old_entry and new_last->next.

2. These helpers cache the result of cmpxchg() and read the initial
   value of head->first before the main loop. I do not think this
   makes sense. In the likely case cmpxchg() succeeds, otherwise
   it doesn't hurt to reload head->first.

   I think it would be better to simplify the code and simply read
   ->first before cmpxchg().
Signed-off-by: default avatarOleg Nesterov <oleg@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrey Vagin <avagin@openvz.org>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Huang Ying <ying.huang@intel.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent 4f5e65a1
...@@ -151,18 +151,13 @@ static inline struct llist_node *llist_next(struct llist_node *node) ...@@ -151,18 +151,13 @@ static inline struct llist_node *llist_next(struct llist_node *node)
*/ */
static inline bool llist_add(struct llist_node *new, struct llist_head *head) static inline bool llist_add(struct llist_node *new, struct llist_head *head)
{ {
struct llist_node *entry, *old_entry; struct llist_node *first;
entry = head->first; do {
for (;;) { new->next = first = ACCESS_ONCE(head->first);
old_entry = entry; } while (cmpxchg(&head->first, first, new) != first);
new->next = entry;
entry = cmpxchg(&head->first, old_entry, new); return !first;
if (entry == old_entry)
break;
}
return old_entry == NULL;
} }
/** /**
......
...@@ -39,18 +39,13 @@ ...@@ -39,18 +39,13 @@
bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last, bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
struct llist_head *head) struct llist_head *head)
{ {
struct llist_node *entry, *old_entry; struct llist_node *first;
entry = head->first; do {
for (;;) { new_last->next = first = ACCESS_ONCE(head->first);
old_entry = entry; } while (cmpxchg(&head->first, first, new_first) != first);
new_last->next = entry;
entry = cmpxchg(&head->first, old_entry, new_first);
if (entry == old_entry)
break;
}
return old_entry == NULL; return !first;
} }
EXPORT_SYMBOL_GPL(llist_add_batch); EXPORT_SYMBOL_GPL(llist_add_batch);
......
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