Re: [PATCH-cgroup] cgroup: Move rcu_head up near the top of cgroup_root

[Waiman Long]

On 12/6/23 23:51, Yosry Ahmed wrote:
> On Wed, Dec 6, 2023 at 8:38 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> > Commit d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU
> > safe") adds a new rcu_head to the cgroup_root structure and kvfree_rcu()
> > for freeing the cgroup_root.
> >
> > The use of kvfree_rcu(), however, has the limitation that the offset of
> > the rcu_head structure within the larger data structure cannot exceed
> > 4096 or the compilation will fail. By putting rcu_head below the cgroup
> > structure, any change to the cgroup structure that makes it larger has
> > the risk of build failure. Commit 77070eeb8821 ("cgroup: Avoid false
> > cacheline sharing of read mostly rstat_cpu") happens to be the commit
> > that breaks it even though it is not its fault. Fix it by moving the
> > rcu_head structure up before the cgroup structure.
> >
> > Fixes: d23b5c577715 ("cgroup: Make operations on the cgroup root_list RCU safe")
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > ---
> >   include/linux/cgroup-defs.h | 8 ++++----
> >   1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> > index 5a97ea95b564..45359969d8cf 100644
> > --- a/include/linux/cgroup-defs.h
> > +++ b/include/linux/cgroup-defs.h
> > @@ -562,6 +562,10 @@ struct cgroup_root {
> >          /* Unique id for this hierarchy. */
> >          int hierarchy_id;
> >
> > +       /* A list running through the active hierarchies */
> > +       struct list_head root_list;
> > +       struct rcu_head rcu;
> > +
> Perhaps the comment should mention the placement requirements, and
> maybe a pointer to wherever it is specified that the offset of struct
> rcu_head should not exceed 4096?

Fair. I will update the patch description to give more details about 
that. It is new to me too before I got a compilation failure report in 
linux-next.

Cheers,
Longman

>