linux-kernel - Re: [PATCH 3/3] cgroup: Remove unused cgroup_subsys::post

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Message-ID: <f7b5caf9-ab30-4b51-b3c0-9e3cdfdc2b2a@redhat.com>
Date: Thu, 4 Sep 2025 11:17:12 -0400
From: Waiman Long <llong@...hat.com>
To: Chuyi Zhou <zhouchuyi@...edance.com>, tj@...nel.org, mkoutny@...e.com,
 hannes@...xchg.org
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] cgroup: Remove unused cgroup_subsys::post_attach

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@...edance.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@...hat.com>