[PATCH v17 46/47] SUNRPC: relocate struct rcu_head to the first field of struct rpc_xprt

Byungchul Park
2025-10-02T08:14:09Z

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