[PATCH] llist: Fix missing memory barrier

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

A smp_read_barrier_depends() appears to be missing in llist_del_first().
It should only matter for Alpha in practice. Adding it after the check
of entry against NULL allows skipping the barrier in a common case.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
CC: Huang Ying <ying.huang@intel.com>
CC: Paul McKenney <paulmck@linux.vnet.ibm.com>
CC: David Howells <dhowells@redhat.com>
---
 lib/llist.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/lib/llist.c b/lib/llist.c
index f76196d..72861f3 100644
--- a/lib/llist.c
+++ b/lib/llist.c
@@ -26,6 +26,7 @@
 #include <linux/export.h>
 #include <linux/interrupt.h>
 #include <linux/llist.h>
+#include <asm/barrier.h>
 
 
 /**
@@ -72,6 +73,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
 		if (entry == NULL)
 			return NULL;
 		old_entry = entry;
+		/*
+		 * Load entry before entry->next. Matches the implicit
+		 * memory barrier before the cmpxchg in llist_add_batch(),
+		 * which ensures entry->next is stored before entry.
+		 */
+		smp_read_barrier_depends();
 		next = entry->next;
 		entry = cmpxchg(&head->first, old_entry, next);
 		if (entry == old_entry)
-- 
2.1.4