linux-kernel - Re: [PATCH v4 3/5] pid: sprinkle tasklist

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Message-ID: <CAGudoHGPAQYwVUEYgxMYZ808=wAO3RteLsqNgG21pvE8Gfs_eQ@mail.gmail.com>
Date: Wed, 5 Feb 2025 21:34:18 +0100
From: Mateusz Guzik <mjguzik@...il.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, Mateusz Guzik <mjguzik@...il.com>, ebiederm@...ssion.com, 
	oleg@...hat.com, brauner@...nel.org, akpm@...ux-foundation.org, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/5] pid: sprinkle tasklist_lock asserts

On Wed, Feb 5, 2025 at 9:27 PM Liam R. Howlett <Liam.Howlett@...cle.com> wrote:
>
> * Mateusz Guzik <mjguzik@...il.com> [250205 14:33]:
>
> If a patch is worth doing, it's worth explaining.  I'm surprised you
> didn't add a comment after the last revision missed a comment on patch
> 2/6.  This is literally in the submitting patches document [1].
>
> I don't mean to delay this series, but I do want to know why things are
> done when I'm hunting through git logs.  Having a change log isn't
> optional, and now you know that Andrews script won't fix this problem
> [2].
>
> I see you are upset by this considering the terse and lack of
> punctuation in patch 2, but please try to understand these comments
> serve a purpose in maintaining the code years later.
>

I'm not upset.

For this specific case I don't know what can be written in the body
given the really self-explanatory nature of the change, other than to
spell it out(?).

Does this work for you:
The routines need to be called with the tasklist_lock, the asserts
validate at runtime that this holds.

I also git log a lot and like to know what's up, to that end I
appreciate *short* commit messages so that I know there is nothing
more to the patch than meets the eye. In particular if there is
nothing of value to add in the body, I appreciate if there is none.

But that's me, I'm not going to insist one way or the other.

> [1]. https://www.kernel.org/doc/html/v6.12/process/submitting-patches.html#describe-your-changes
> [2]. https://lore.kernel.org/all/20250203175128.80319b42c9739f0d420080a4@linux-foundation.org/
>
> > Reviewed-by: Oleg Nesterov <oleg@...hat.com>
> > Signed-off-by: Mateusz Guzik <mjguzik@...il.com>
> > ---
> >  kernel/pid.c | 15 ++++++++++++---
> >  1 file changed, 12 insertions(+), 3 deletions(-)
> >
> > diff --git a/kernel/pid.c b/kernel/pid.c
> > index 924084713be8..2ae872f689a7 100644
> > --- a/kernel/pid.c
> > +++ b/kernel/pid.c
> > @@ -339,17 +339,23 @@ static struct pid **task_pid_ptr(struct task_struct *task, enum pid_type type)
> >   */
> >  void attach_pid(struct task_struct *task, enum pid_type type)
> >  {
> > -     struct pid *pid = *task_pid_ptr(task, type);
> > +     struct pid *pid;
> > +
> > +     lockdep_assert_held_write(&tasklist_lock);
> > +
> > +     pid = *task_pid_ptr(task, type);
> >       hlist_add_head_rcu(&task->pid_links[type], &pid->tasks[type]);
> >  }
> >
> >  static void __change_pid(struct task_struct *task, enum pid_type type,
> >                       struct pid *new)
> >  {
> > -     struct pid **pid_ptr = task_pid_ptr(task, type);
> > -     struct pid *pid;
> > +     struct pid **pid_ptr, *pid;
> >       int tmp;
> >
> > +     lockdep_assert_held_write(&tasklist_lock);
> > +
> > +     pid_ptr = task_pid_ptr(task, type);
> >       pid = *pid_ptr;
> >
> >       hlist_del_rcu(&task->pid_links[type]);
> > @@ -386,6 +392,8 @@ void exchange_tids(struct task_struct *left, struct task_struct *right)
> >       struct hlist_head *head1 = &pid1->tasks[PIDTYPE_PID];
> >       struct hlist_head *head2 = &pid2->tasks[PIDTYPE_PID];
> >
> > +     lockdep_assert_held_write(&tasklist_lock);
> > +
> >       /* Swap the single entry tid lists */
> >       hlists_swap_heads_rcu(head1, head2);
> >
> > @@ -403,6 +411,7 @@ void transfer_pid(struct task_struct *old, struct task_struct *new,
> >                          enum pid_type type)
> >  {
> >       WARN_ON_ONCE(type == PIDTYPE_PID);
> > +     lockdep_assert_held_write(&tasklist_lock);
> >       hlist_replace_rcu(&old->pid_links[type], &new->pid_links[type]);
> >  }
> >
> > --
> > 2.43.0
> >
> >



-- 
Mateusz Guzik <mjguzik gmail.com>