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
