Re: [patch 7/9] signals: Fix more rcu assumptions

[Oleg Nesterov <oleg@redhat.com>]

On 12/10, Thomas Gleixner wrote:
>
> 1) Remove the misleading comment in __sigqueue_alloc() which claims
>    that holding a spinlock is equivalent to rcu_read_lock().
>
> 2) Wrap the __send_signal() call in send_signal() into a rcu read side
>    critical section to guarantee that the __sigqueue_alloc()
>    requirement is met in any case.
> ...
>  static int send_signal(int sig, struct siginfo *info, struct task_struct *t,
>  			int group)
>  {
> -	int from_ancestor_ns = 0;
> +	int ret, from_ancestor_ns = 0;
>
>  #ifdef CONFIG_PID_NS
>  	if (!is_si_special(info) && SI_FROMUSER(info) &&
> @@ -954,7 +953,11 @@ static int send_signal(int sig, struct s
>  		from_ancestor_ns = 1;
>  #endif
>
> -	return __send_signal(sig, info, t, group, from_ancestor_ns);
> +	rcu_read_lock();
> +	ret = __send_signal(sig, info, t, group, from_ancestor_ns);
> +	rcu_read_unlock();

But, without a comment it is very unobvious why do we need rcu_read_lock().

Perhaps it is better to modify __sigqueue_alloc() instead? It can take
rcu_lock() around cred->user itself.

Oleg.