From a3127336b71f6833d1483c856dce91fe558dc3a9 Mon Sep 17 00:00:00 2001 From: Huang Ying Date: Thu, 8 Sep 2011 14:00:44 +0800 Subject: [PATCH] llist: Move cpu_relax() to after the cmpxchg() If in llist_add()/etc. functions the first cmpxchg() call succeeds, it is not necessary to use cpu_relax() before the cmpxchg(). So cpu_relax() in a busy loop involving cmpxchg() should go after cmpxchg() instead of before that. This patch fixes this for all involved llist functions. Signed-off-by: Huang Ying Acked-by: Mathieu Desnoyers Signed-off-by: Peter Zijlstra Link: http://lkml.kernel.org/r/1315461646-1379-4-git-send-email-ying.huang@intel.com Signed-off-by: Ingo Molnar --- include/linux/llist.h | 7 +++++-- lib/llist.c | 14 ++++++++++---- 2 files changed, 15 insertions(+), 6 deletions(-) diff --git a/include/linux/llist.h b/include/linux/llist.h index 65fca1cbf514..ca91875286bf 100644 --- a/include/linux/llist.h +++ b/include/linux/llist.h @@ -148,11 +148,14 @@ static inline void llist_add(struct llist_node *new, struct llist_head *head) struct llist_node *entry, *old_entry; entry = head->first; - do { + for (;;) { old_entry = entry; new->next = entry; + entry = cmpxchg(&head->first, old_entry, new); + if (entry == old_entry) + break; cpu_relax(); - } while ((entry = cmpxchg(&head->first, old_entry, new)) != old_entry); + } } /** diff --git a/lib/llist.c b/lib/llist.c index b445f2c8596a..6c69f1d14c4b 100644 --- a/lib/llist.c +++ b/lib/llist.c @@ -41,11 +41,14 @@ void llist_add_batch(struct llist_node *new_first, struct llist_node *new_last, struct llist_node *entry, *old_entry; entry = head->first; - do { + for (;;) { old_entry = entry; new_last->next = entry; + entry = cmpxchg(&head->first, old_entry, new_first); + if (entry == old_entry) + break; cpu_relax(); - } while ((entry = cmpxchg(&head->first, old_entry, new_first)) != old_entry); + } } EXPORT_SYMBOL_GPL(llist_add_batch); @@ -68,13 +71,16 @@ struct llist_node *llist_del_first(struct llist_head *head) struct llist_node *entry, *old_entry, *next; entry = head->first; - do { + for (;;) { if (entry == NULL) return NULL; old_entry = entry; next = entry->next; + entry = cmpxchg(&head->first, old_entry, next); + if (entry == old_entry) + break; cpu_relax(); - } while ((entry = cmpxchg(&head->first, old_entry, next)) != old_entry); + } return entry; }