Re: [PATCH] kthread: update comment for __to_kthread

[Jiazi Li]

On Thu, Jun 19, 2025 at 12:34:20PM -0500, Eric W. Biederman wrote:
> Jiazi Li <jqqlijiazi@xxxxxxxxx> 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
> 
> 
Ok, thanks for your suggestion.
> > Signed-off-by: Jiazi Li <jqqlijiazi@xxxxxxxxx>
> > Signed-off-by: mingzhu.wang <mingzhu.wang@xxxxxxxxxxxxx>
> > ---
> >  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)
> >  {