While compiling Linux kernel with DEPT on, the following error was
observed:

   ./include/linux/rcupdate.h:1084:17: note: in expansion of macro
   ‘BUILD_BUG_ON’
   1084 | BUILD_BUG_ON(offsetof(typeof(*(ptr)), rhf) >= 4096);	\
        | ^~~~~~~~~~~~
   ./include/linux/rcupdate.h:1047:29: note: in expansion of macro
   'kvfree_rcu_arg_2'
   1047 | #define kfree_rcu(ptr, rhf) kvfree_rcu_arg_2(ptr, rhf)
        |                             ^~~~~~~~~~~~~~~~
   net/sunrpc/xprt.c:1856:9: note: in expansion of macro 'kfree_rcu'
   1856 | kfree_rcu(xprt, rcu);
        | ^~~~~~~~~
    CC net/kcm/kcmproc.o
   make[4]: *** [scripts/Makefile.build:203: net/sunrpc/xprt.o] Error 1

Since kfree_rcu() assumes 'offset of struct rcu_head in a rcu-managed
struct < 4096', the offest of struct rcu_head in struct rpc_xprt should
not exceed 4096 but does, due to the debug information added by DEPT.

Relocate struct rcu_head to the first field of struct rpc_xprt from an
arbitrary location to avoid the issue and meet the assumption.

Reported-by: Yunseong Kim <ysk@kzalloc.com>
Signed-off-by: Byungchul Park <byungchul@sk.com>
---
 include/linux/sunrpc/xprt.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/sunrpc/xprt.h b/include/linux/sunrpc/xprt.h
index f46d1fb8f71a..666e42a17a31 100644
--- a/include/linux/sunrpc/xprt.h
+++ b/include/linux/sunrpc/xprt.h
@@ -211,6 +211,14 @@ enum xprt_transports {
 
 struct rpc_sysfs_xprt;
 struct rpc_xprt {
+	/*
+	 * Place struct rcu_head within the first 4096 bytes of struct
+	 * rpc_xprt if sizeof(struct rpc_xprt) > 4096, so that
+	 * kfree_rcu() can simply work assuming that.  See the comment
+	 * in kfree_rcu().
+	 */
+	struct rcu_head		rcu;
+
 	struct kref		kref;		/* Reference count */
 	const struct rpc_xprt_ops *ops;		/* transport methods */
 	unsigned int		id;		/* transport id */
@@ -317,7 +325,6 @@ struct rpc_xprt {
 #if IS_ENABLED(CONFIG_SUNRPC_DEBUG)
 	struct dentry		*debugfs;		/* debugfs directory */
 #endif
-	struct rcu_head		rcu;
 	const struct xprt_class	*xprt_class;
 	struct rpc_sysfs_xprt	*xprt_sysfs;
 	bool			main; /*mark if this is the 1st transport */
-- 
2.17.1