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Commit fb4214db50b00558cc6e274c88b3f7325068e942

Authored by Oleg Nesterov
Committed by Al Viro
1 parent 4f5e65a1cc
Exists in smarc-imx_3.14.28_1.0.0_ga and in 1 other branch imx_3.14.28_1.0.0_ga

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: 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>

Showing 2 changed files with 12 additions and 22 deletions Side-by-side Diff

include/linux/llist.h
Diff comments   View file @ fb4214d
... ... @@ -151,18 +151,13 @@
151 151 */
152 152 static inline bool llist_add(struct llist_node *new, struct llist_head *head)
153 153 {
154   - struct llist_node *entry, *old_entry;
  154 + struct llist_node *first;
155 155  
156   - entry = head->first;
157   - for (;;) {
158   - old_entry = entry;
159   - new->next = entry;
160   - entry = cmpxchg(&head->first, old_entry, new);
161   - if (entry == old_entry)
162   - break;
163   - }
  156 + do {
  157 + new->next = first = ACCESS_ONCE(head->first);
  158 + } while (cmpxchg(&head->first, first, new) != first);
164 159  
165   - return old_entry == NULL;
  160 + return !first;
166 161 }
167 162  
168 163 /**
... ... @@ -39,18 +39,13 @@
39 39 bool llist_add_batch(struct llist_node *new_first, struct llist_node *new_last,
40 40 struct llist_head *head)
41 41 {
42   - struct llist_node *entry, *old_entry;
  42 + struct llist_node *first;
43 43  
44   - entry = head->first;
45   - for (;;) {
46   - old_entry = entry;
47   - new_last->next = entry;
48   - entry = cmpxchg(&head->first, old_entry, new_first);
49   - if (entry == old_entry)
50   - break;
51   - }
  44 + do {
  45 + new_last->next = first = ACCESS_ONCE(head->first);
  46 + } while (cmpxchg(&head->first, first, new_first) != first);
52 47  
53   - return old_entry == NULL;
  48 + return !first;
54 49 }
55 50 EXPORT_SYMBOL_GPL(llist_add_batch);
56 51