Re: [PATCH] pid: document that do_each_pid_task() needs tasklist_lock

[David Laight <david.laight.linux@gmail.com>]

On Sun, 11 Oct 2026 16:16:51 +0200
Oleg Nesterov <oleg@redhat.com> wrote:

> do_each_pid_task() looks "obviously rcu-safe". This confuses people
> and we already have 2 buggy users: send_sigio() and send_sigurg().
> 
> The reasons why it is not are not obvious at all, add the comment.
> 
> Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> ---
>  include/linux/pid.h | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/include/linux/pid.h b/include/linux/pid.h
> index ddaef0bbc8ba..f5476d7140a3 100644
> --- a/include/linux/pid.h
> +++ b/include/linux/pid.h
> @@ -187,6 +187,15 @@ static inline pid_t pid_nr(struct pid *pid)
>  pid_t pid_nr_ns(struct pid *pid, struct pid_namespace *ns);
>  pid_t pid_vnr(struct pid *pid);
>  
> +/*
> + * Needs tasklist_lock if type > PIDTYPE_TGID. A lockless reader can:

Think I'd be more explicit, so:
	... if type isn't PIDTYPE_PID or PIDTYPE_TGID.
David
 
> + *
> + * - race with a change_pid() that moves the task to another hlist,
> + *   and continue to scan the wrong list.
> + *
> + * - race with a copy_process() that adds a new task to the start of hlist,
> + *   and miss the new child.
> + */
>  #define do_each_pid_task(pid, type, task)				\
>  	do {								\
>  		if ((pid) != NULL)					\