Re: [PATCH 4/7] pid: modify task_tgid_nr to work without task->tgid.

[Oleg Nesterov <oleg@redhat.com>]

On 01/23, Richard Guy Briggs wrote:
>
> task->tgid is an error prone construct and results in duplicate maintenance.
> Start it's demise by modifying task_tgid_nr to not use it.

Well, I disagree.

Yes I agree that ->tgid should probably die. But this change itself doesn't
help, it only makes task_tgid_nr() slower. We need to convert other users
first, then consider this change along with ->tgid removal.

Besides, this is not that simple and the patch doesn't look right:

>  static inline pid_t task_tgid_nr(struct task_struct *tsk)
>  {
> -	return tsk->tgid;
> +	return pid_nr(task_tgid(tsk));
>  }

And what protect task_tgid? This is racy.

The race is very unlikely, pid_nr() will likely hit pid == NULL if tsk
exits. But still it can use the freed/unmapped/reused memory.

And even if we add rcu_read_lock() the patch will add the semantics change,
task_tgid_nr() can return 0 if tsk has already exited. At least this should
be documented, but you also need to audit the users.

Oleg.