Re: [PATCH v4 3/5] pid: sprinkle tasklist_lock asserts

["Liam R. Howlett" <Liam.Howlett@oracle.com>]

* Mateusz Guzik <mjguzik@gmail.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.

[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@redhat.com>
> Signed-off-by: Mateusz Guzik <mjguzik@gmail.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
> 
>