linux-kernel - [PATCH v3 15/33] radix-tree: Move rcu_head into a union with private

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Message-Id: <1480369871-5271-16-git-send-email-mawilcox@linuxonhyperv.com>
Date:   Mon, 28 Nov 2016 13:50:19 -0800
From:   Matthew Wilcox <mawilcox@...uxonhyperv.com>
To:     linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <koct9i@...il.com>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>
Cc:     Matthew Wilcox <willy@...radead.org>, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: [PATCH v3 15/33] radix-tree: Move rcu_head into a union with private_list

From: Matthew Wilcox <willy@...radead.org>

I want to be able to reference node->parent after freeing node.
Currently node->parent is in a union with rcu_head, so it is overwritten
when the node is put on the RCU list.  We know that private_list is not
referenced after the node is freed, so it is safe for these two members
to share space.

Signed-off-by: Matthew Wilcox <willy@...radead.org>
---
 include/linux/radix-tree.h | 14 ++++----------
 lib/radix-tree.c           |  1 +
 2 files changed, 5 insertions(+), 10 deletions(-)

diff --git a/include/linux/radix-tree.h b/include/linux/radix-tree.h
index 7444860..d04073a 100644
--- a/include/linux/radix-tree.h
+++ b/include/linux/radix-tree.h
@@ -85,18 +85,12 @@ struct radix_tree_node {
 	unsigned char	offset;		/* Slot offset in parent */
 	unsigned char	count;		/* Total entry count */
 	unsigned char	exceptional;	/* Exceptional entry count */
+	struct radix_tree_node *parent;		/* Used when ascending tree */
+	void *private_data;			/* For tree user */
 	union {
-		struct {
-			/* Used when ascending tree */
-			struct radix_tree_node *parent;
-			/* For tree user */
-			void *private_data;
-		};
-		/* Used when freeing node */
-		struct rcu_head	rcu_head;
+		struct list_head private_list;	/* For tree user */
+		struct rcu_head	rcu_head;	/* Used when freeing node */
 	};
-	/* For tree user */
-	struct list_head private_list;
 	void __rcu	*slots[RADIX_TREE_MAP_SIZE];
 	unsigned long	tags[RADIX_TREE_MAX_TAGS][RADIX_TREE_TAG_LONGS];
 };
diff --git a/lib/radix-tree.c b/lib/radix-tree.c
index b329056..3f49417 100644
--- a/lib/radix-tree.c
+++ b/lib/radix-tree.c
@@ -324,6 +324,7 @@ static void radix_tree_node_rcu_free(struct rcu_head *head)
 		tag_clear(node, i, 0);
 
 	node->slots[0] = NULL;
+	INIT_LIST_HEAD(&node->private_list);
 
 	kmem_cache_free(radix_tree_node_cachep, node);
 }
-- 
2.10.2