[PATCH] mm/slab.c: add node spinlock protect in __cache_free_alien

[<qiang.zhang@xxxxxxxxxxxxx>]

From: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>

We should add node spinlock protect "n->alien" which may be
assigned to NULL in cpuup_canceled func. cause address access
exception.

Fixes: 18bf854117c6 ("slab: use get_node() and kmem_cache_node() functions")
Signed-off-by: Zhang Qiang <qiang.zhang@xxxxxxxxxxxxx>
---
 mm/slab.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/mm/slab.c b/mm/slab.c
index a89633603b2d..290523c90b4e 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -759,8 +759,10 @@ static int __cache_free_alien(struct kmem_cache *cachep, void *objp,
 
 	n = get_node(cachep, node);
 	STATS_INC_NODEFREES(cachep);
+	spin_lock(&n->list_lock);
 	if (n->alien && n->alien[page_node]) {
 		alien = n->alien[page_node];
+		spin_unlock(&n->list_lock);
 		ac = &alien->ac;
 		spin_lock(&alien->lock);
 		if (unlikely(ac->avail == ac->limit)) {
@@ -769,14 +771,15 @@ static int __cache_free_alien(struct kmem_cache *cachep, void *objp,
 		}
 		ac->entry[ac->avail++] = objp;
 		spin_unlock(&alien->lock);
-		slabs_destroy(cachep, &list);
 	} else {
+		spin_unlock(&n->list_lock);
 		n = get_node(cachep, page_node);
 		spin_lock(&n->list_lock);
 		free_block(cachep, &objp, 1, page_node, &list);
 		spin_unlock(&n->list_lock);
-		slabs_destroy(cachep, &list);
 	}
+
+	slabs_destroy(cachep, &list);
 	return 1;
 }
 
-- 
2.26.2