Re: [PATCH 06/26] rlimit: Remove unnecessary grab of tasklist_lock

[Oleg Nesterov]

Hi Eric,

I'll try very much to read this series tomorrow, can't do this today...

On 06/06, Eric W. Biederman wrote:
>
> @@ -1380,13 +1380,6 @@ int do_prlimit(struct task_struct *tsk, unsigned int resource,
>  			return -EPERM;
>  	}
>  
> -	/* protect tsk->signal and tsk->sighand from disappearing */
> -	read_lock(&tasklist_lock);
> -	if (!tsk->sighand) {
> -		retval = -ESRCH;
> -		goto out;
> -	}

Yes, the comment is wrong.

However we do need read_lock(tasklist_lock) to access ->group_leader. And the
->sighand != NULL check ensures that ->group_leader is the valid pointer.

Also, update_rlimit_cpu() is not safe without tasklist / sighand-check.

We can probably change this code to rely on rcu.

Oleg.