[PATCH 01/14] xattr: add rcu_head and rhash_head to struct simple_xattr

[Christian Brauner <brauner@kernel.org>]

In preparation for converting simple_xattrs from rbtree to rhashtable,
add rhash_head and rcu_head members to struct simple_xattr. The
rhashtable implementation will use rhash_head for hash table linkage
and RCU-based lockless reads, requiring that replaced or removed xattr
entries be freed via call_rcu() rather than immediately.

Add simple_xattr_free_rcu() which schedules RCU-deferred freeing of an
xattr entry.  This will be used by callers of simple_xattr_set() once
they switch to the rhashtable-based xattr store.

No functional changes.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 fs/xattr.c            | 23 +++++++++++++++++++++++
 include/linux/xattr.h |  4 ++++
 2 files changed, 27 insertions(+)

diff --git a/fs/xattr.c b/fs/xattr.c
index 3e49e612e1ba..9cbb1917bcb2 100644
--- a/fs/xattr.c
+++ b/fs/xattr.c
@@ -1197,6 +1197,29 @@ void simple_xattr_free(struct simple_xattr *xattr)
 	kvfree(xattr);
 }
 
+static void simple_xattr_rcu_free(struct rcu_head *head)
+{
+	struct simple_xattr *xattr;
+
+	xattr = container_of(head, struct simple_xattr, rcu);
+	simple_xattr_free(xattr);
+}
+
+/**
+ * simple_xattr_free_rcu - free an xattr object after an RCU grace period
+ * @xattr: the xattr object
+ *
+ * Schedule RCU-deferred freeing of an xattr entry. This is used by
+ * rhashtable-based callers of simple_xattr_set() that replace or remove
+ * an existing entry while concurrent RCU readers may still be accessing
+ * it.
+ */
+void simple_xattr_free_rcu(struct simple_xattr *xattr)
+{
+	if (xattr)
+		call_rcu(&xattr->rcu, simple_xattr_rcu_free);
+}
+
 /**
  * simple_xattr_alloc - allocate new xattr object
  * @value: value of the xattr object
diff --git a/include/linux/xattr.h b/include/linux/xattr.h
index 64e9afe7d647..1328f2bfd2ce 100644
--- a/include/linux/xattr.h
+++ b/include/linux/xattr.h
@@ -16,6 +16,7 @@
 #include <linux/types.h>
 #include <linux/spinlock.h>
 #include <linux/mm.h>
+#include <linux/rhashtable-types.h>
 #include <linux/user_namespace.h>
 #include <uapi/linux/xattr.h>
 
@@ -112,6 +113,8 @@ struct simple_xattrs {
 
 struct simple_xattr {
 	struct rb_node rb_node;
+	struct rhash_head hash_node;
+	struct rcu_head rcu;
 	char *name;
 	size_t size;
 	char value[];
@@ -122,6 +125,7 @@ void simple_xattrs_free(struct simple_xattrs *xattrs, size_t *freed_space);
 size_t simple_xattr_space(const char *name, size_t size);
 struct simple_xattr *simple_xattr_alloc(const void *value, size_t size);
 void simple_xattr_free(struct simple_xattr *xattr);
+void simple_xattr_free_rcu(struct simple_xattr *xattr);
 int simple_xattr_get(struct simple_xattrs *xattrs, const char *name,
 		     void *buffer, size_t size);
 struct simple_xattr *simple_xattr_set(struct simple_xattrs *xattrs,

-- 
2.47.3