linux-kernel - Re: [PATCH] kthread: update comment for __to

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Message-ID: <877c17zlcz.fsf@email.froward.int.ebiederm.org>
Date: Thu, 19 Jun 2025 12:34:20 -0500
From: "Eric W. Biederman" <ebiederm@...ssion.com>
To: Jiazi Li <jqqlijiazi@...il.com>
Cc: Peter Zijlstra <peterz@...radead.org>,  Andrew Morton
 <akpm@...ux-foundation.org>,  linux-kernel@...r.kernel.org,
  "mingzhu.wang" <mingzhu.wang@...nssion.com>
Subject: Re: [PATCH] kthread: update comment for __to_kthread

Jiazi Li <jqqlijiazi@...il.com> writes:

> With commit 343f4c49f243 ("kthread: Don't allocate kthread_struct
> for init and umh") and commit 753550eb0ce1 ("fork: Explicitly set
> PF_KTHREAD"), umh task no longer have struct kthread and PF_KTHREAD flag.
> Io_uring worker thread does not have PF_KTHREAD flag, but worker_private
> is not null, so update comment for __to_kthread.

I agree the comment is a bit dated and can use being updated.

Removing the portion describing the exceptions for init and umh is good
as they no longer apply.

However I don't think we want to go on about io_uring and whatever
else winds up using p->worker_private as exceptions.  That will just
mean the comment goes stale next time there is a change.

Can you update the comment to describe what the current rules
are to detect is something is a kthread?

Perhaps:
  *
  * When "(p->flags & PF_KTHREAD)" is set the task is a kthread and will
  * always remain a kthread.  For kthreads p->worker_private always
  * points to a struct kthread.  For tasks that are not kthreads
  * p->worker_private is used to point to other things.
  *
  * Return NULL for any task that is not a kthread.
  */

Eric

> Signed-off-by: Jiazi Li <jqqlijiazi@...il.com>
> Signed-off-by: mingzhu.wang <mingzhu.wang@...nssion.com>
> ---
>  kernel/kthread.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/kthread.c b/kernel/kthread.c
> index 85fc068f0083..810cc244b81c 100644
> --- a/kernel/kthread.c
> +++ b/kernel/kthread.c
> @@ -88,13 +88,9 @@ static inline struct kthread *to_kthread(struct task_struct *k)
>  /*
>   * Variant of to_kthread() that doesn't assume @p is a kthread.
>   *
> - * Per construction; when:
> - *
> - *   (p->flags & PF_KTHREAD) && p->worker_private
> - *
> - * the task is both a kthread and struct kthread is persistent. However
> - * PF_KTHREAD on it's own is not, kernel_thread() can exec() (See umh.c and
> - * begin_new_exec()).
> + * For io_uring worker thread, p->worker_private point to io_worker
> + * struct, but they do not have PF_KTHREAD flag.
> + * Return NULL for such tasks.
>   */
>  static inline struct kthread *__to_kthread(struct task_struct *p)
>  {