On Fri, Dec 05, 2025 at 04:27:52AM -0500, Jeff Layton wrote:
> On Fri, 2025-12-05 at 16:18 +0900, Byungchul Park wrote:
> > 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 <[email protected]>
> > Signed-off-by: Byungchul Park <[email protected]>
> > ---
> > 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 */
>
> Seems fine to me.
>
> Reviewed-by: Jeff Layton <[email protected]>
Thank you, Jeff.
Byungchul
