From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754404Ab3FORff (ORCPT ); Sat, 15 Jun 2013 13:35:35 -0400 Received: from mx1.redhat.com ([209.132.183.28]:42661 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754112Ab3FORey (ORCPT ); Sat, 15 Jun 2013 13:34:54 -0400 Date: Sat, 15 Jun 2013 19:30:43 +0200 From: Oleg Nesterov To: Andrew Morton Cc: Al Viro , Andrey Vagin , "Eric W. Biederman" , David Howells , Huang Ying , Peter Zijlstra , linux-kernel@vger.kernel.org Subject: [PATCH 2/3] llist: fix/simplify llist_add() and llist_add_batch() Message-ID: <20130615173043.GA15065@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130615172959.GA14656@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 --- include/linux/llist.h | 19 +++++++------------ lib/llist.c | 15 +++++---------- 2 files changed, 12 insertions(+), 22 deletions(-) diff --git a/include/linux/llist.h b/include/linux/llist.h index a5199f6..3e2b969 100644 --- a/include/linux/llist.h +++ b/include/linux/llist.h @@ -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; - - entry = head->first; - for (;;) { - old_entry = entry; - new->next = entry; - entry = cmpxchg(&head->first, old_entry, new); - if (entry == old_entry) - break; - } - - return old_entry == NULL; + struct llist_node *first; + + do { + new->next = first = ACCESS_ONCE(head->first); + } while (cmpxchg(&head->first, first, new) != first); + + return !first; } /** diff --git a/lib/llist.c b/lib/llist.c index 4a15115..4a70d12 100644 --- a/lib/llist.c +++ b/lib/llist.c @@ -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); -- 1.5.5.1