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

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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: Oleg 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Oleg Nesterov 2013-07-08 14:24:18 -07:00 committed by Al Viro
parent 4f5e65a1cc
commit fb4214db50
2 changed files with 10 additions and 20 deletions
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15
include/linux/llist.h
View file

@ -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)
{
struct llist_node *entry, *old_entry;
struct llist_node *first;
entry = head->first;
for (;;) {
old_entry = entry;
new->next = entry;
entry = cmpxchg(&head->first, old_entry, new);
if (entry == old_entry)
break;
}
do {
new->next = first = ACCESS_ONCE(head->first);
} while (cmpxchg(&head->first, first, new) != first);
return old_entry == NULL;
return !first;
}
/**

15
lib/llist.c
View file

@ -39,18 +39,13 @@
bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
struct llist_head *head)
{
struct llist_node *entry, *old_entry;
struct llist_node *first;
entry = head->first;
for (;;) {
old_entry = entry;
new_last->next = entry;
entry = cmpxchg(&head->first, old_entry, new_first);
if (entry == old_entry)
break;
}
do {
new_last->next = first = ACCESS_ONCE(head->first);
} while (cmpxchg(&head->first, first, new_first) != first);
return old_entry == NULL;
return !first;
}
EXPORT_SYMBOL_GPL(llist_add_batch);