linux-kernel - [PATCH] llist: Fix missing lockless

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Date:	Fri,  6 Feb 2015 21:08:21 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	David Howells <dhowells@...hat.com>,
	Pranith Kumar <bobby.prani@...il.com>, stable@...r.kernel.org
Subject: [PATCH] llist: Fix missing lockless_dereference()

A lockless_dereference() appears to be missing in llist_del_first().
It should only matter for Alpha in practice.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
CC: Huang Ying <ying.huang@...el.com>
CC: Paul McKenney <paulmck@...ux.vnet.ibm.com>
CC: David Howells <dhowells@...hat.com>
CC: Pranith Kumar <bobby.prani@...il.com>
CC: stable@...r.kernel.org # v3.1+
---
 lib/llist.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/lib/llist.c b/lib/llist.c
index f76196d..f34e176 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 <linux/rcupdate.h>
 
 
 /**
@@ -67,7 +68,12 @@ struct llist_node *llist_del_first(struct llist_head *head)
 {
 	struct llist_node *entry, *old_entry, *next;
 
-	entry = head->first;
+	/*
+	 * 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.
+	 */
+	entry = lockless_dereference(head->first);
 	for (;;) {
 		if (entry == NULL)
 			return NULL;
-- 
2.1.4

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