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 <ying.huang@intel.com>
Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1315461646-1379-4-git-send-email-ying.huang@intel.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/include/linux/llist.h b/include/linux/llist.h
index 65fca1c..ca91875 100644
--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -148,11 +148,14 @@
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 b445f2c..6c69f1d 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -41,11 +41,14 @@
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 *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;
}