[PATCH -mm 3/4] llist, Move cpu_relax after cmpxchg

[Huang Ying <ying.huang@intel.com>]

If the first cmpxchg calling succeeds, it is not necessary to use
cpu_relax before cmpxchg.  So cpu_relax in a busy loop involving
cmpxchg should go after cmpxchg instead of before that.  This patch
fixes this in llist.

Signed-off-by: Huang Ying <ying.huang@intel.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 include/linux/llist.h |   21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

--- a/include/linux/llist.h
+++ b/include/linux/llist.h
@@ -156,11 +156,14 @@ static inline void llist_add(struct llis
 	CHECK_NMI_SAFE_CMPXCHG();
 
 	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);
+	}
 }
 
 /**
@@ -178,11 +181,14 @@ static inline void llist_add_batch(struc
 	CHECK_NMI_SAFE_CMPXCHG();
 
 	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);
+	}
 }
 
 /**
@@ -206,13 +212,16 @@ static inline struct llist_node *llist_d
 	CHECK_NMI_SAFE_CMPXCHG();
 
 	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;
 }