Re: [RFC,PATCH] Use RCU to protect tasklist for unicast signals

[Oleg Nesterov <oleg@tv-sign.ru>]

Paul E. McKenney wrote:
>
> --- linux-2.6.13-rc6/kernel/signal.c	2005-08-08 19:59:24.000000000 -0700
> +++ linux-2.6.13-rc6-tasklistRCU/kernel/signal.c	2005-08-10 08:20:25.000000000 -0700
> @@ -1151,9 +1151,13 @@ int group_send_sig_info(int sig, struct 
>
>  	ret = check_kill_permission(sig, info, p);
>  	if (!ret && sig && p->sighand) {
> +		if (!get_task_struct_rcu(p)) {
> +			return -ESRCH;
> +		}
>  		spin_lock_irqsave(&p->sighand->siglock, flags);
                                      ^^^^^^^
Is it correct?

The caller (kill_proc_info) does not take tasklist_lock anymore.
If p does exec() at this time it can change/free its ->sighand.

fs/exec.c:de_thread()
   773                  write_lock_irq(&tasklist_lock);
   774                  spin_lock(&oldsighand->siglock);
   775                  spin_lock(&newsighand->siglock);
   776
   777                  current->sighand = newsighand;
   778                  recalc_sigpending();
   779
   780                  spin_unlock(&newsighand->siglock);
   781                  spin_unlock(&oldsighand->siglock);
   782                  write_unlock_irq(&tasklist_lock);
   783
   784                  if (atomic_dec_and_test(&oldsighand->count))
   785                          kmem_cache_free(sighand_cachep, oldsighand);

Oleg.