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

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Message-ID: <jamcxevfeuzsqhsj7b5o7jt2avfqpqmcahwadiwnayj6cwqpvf@psksqzyktksg>
Date: Wed, 5 Feb 2025 15:42:43 -0500
From: "Liam R. Howlett" <Liam.Howlett@...cle.com>
To: Mateusz Guzik <mjguzik@...il.com>
Cc: 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

* Mateusz Guzik <mjguzik@...il.com> [250205 15:34]:
> 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.

Good, thanks - that wasn't my intention.

> 
> 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(?).

You could say why you added it?  Is this something that was seen
happening?

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

Well, you are checking this lock because it is protecting something
that's being changed.  You could say "the tasklist_lock protects X, make
sure that it's held"?

> 
> 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>