cgroup_subsys::post_attach callback was introduced in commit 5cf1cacb49ae
("cgroup, cpuset: replace cpuset_post_attach_flush() with
cgroup_subsys->post_attach callback") and only cpuset would use this
callback to wait for the mm migration to complete at the end of
__cgroup_procs_write(). Since the previous patch defer the flush operation
until returning to userspace, no one use this callback now. Remove this
callback from cgroup_subsys.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
---
 include/linux/cgroup-defs.h | 1 -
 kernel/cgroup/cgroup.c      | 4 ----
 2 files changed, 5 deletions(-)

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 6b93a64115fe9..432abdfdb2593 100644
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -746,7 +746,6 @@ struct cgroup_subsys {
 	int (*can_attach)(struct cgroup_taskset *tset);
 	void (*cancel_attach)(struct cgroup_taskset *tset);
 	void (*attach)(struct cgroup_taskset *tset);
-	void (*post_attach)(void);
 	int (*can_fork)(struct task_struct *task,
 			struct css_set *cset);
 	void (*cancel_fork)(struct task_struct *task, struct css_set *cset);
diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 312c6a8b55bb7..75819bb2f1148 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3033,10 +3033,6 @@ void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked
 	put_task_struct(task);
 
 	cgroup_attach_unlock(threadgroup_locked);
-
-	for_each_subsys(ss, ssid)
-		if (ss->post_attach)
-			ss->post_attach();
 }
 
 static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)
-- 
2.20.1

On 9/4/25 3:45 AM, Chuyi Zhou wrote:
> cgroup_subsys::post_attach callback was introduced in commit 5cf1cacb49ae
> ("cgroup, cpuset: replace cpuset_post_attach_flush() with
> cgroup_subsys->post_attach callback") and only cpuset would use this
> callback to wait for the mm migration to complete at the end of
> __cgroup_procs_write(). Since the previous patch defer the flush operation
> until returning to userspace, no one use this callback now. Remove this
> callback from cgroup_subsys.
>
> Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
> ---
>   include/linux/cgroup-defs.h | 1 -
>   kernel/cgroup/cgroup.c      | 4 ----
>   2 files changed, 5 deletions(-)
>
> diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
> index 6b93a64115fe9..432abdfdb2593 100644
> --- a/include/linux/cgroup-defs.h
> +++ b/include/linux/cgroup-defs.h
> @@ -746,7 +746,6 @@ struct cgroup_subsys {
>   	int (*can_attach)(struct cgroup_taskset *tset);
>   	void (*cancel_attach)(struct cgroup_taskset *tset);
>   	void (*attach)(struct cgroup_taskset *tset);
> -	void (*post_attach)(void);
>   	int (*can_fork)(struct task_struct *task,
>   			struct css_set *cset);
>   	void (*cancel_fork)(struct task_struct *task, struct css_set *cset);
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 312c6a8b55bb7..75819bb2f1148 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -3033,10 +3033,6 @@ void cgroup_procs_write_finish(struct task_struct *task, bool threadgroup_locked
>   	put_task_struct(task);
>   
>   	cgroup_attach_unlock(threadgroup_locked);
> -
> -	for_each_subsys(ss, ssid)
> -		if (ss->post_attach)
> -			ss->post_attach();
>   }
>   
>   static void cgroup_print_ss_mask(struct seq_file *seq, u16 ss_mask)

Note that we may have to add it back in the future if a new use case 
comes up.

Acked-by:  Waiman Long <longman@redhat.com>