Subject: Re: [PATCH net-next-2.6] fasync: RCU locking

From: Lai Jiangshan <[email-masked]>
Date: Wed, 14 Apr 2010 16:36:13 +0800

Eric Dumazet wrote:
> -void __kill_fasync(struct fasync_struct *fa, int sig, int band)
> +/*
> + * rcu_read_lock() is held
> + */
> +static void kill_fasync_rcu(struct fasync_struct *fa, int sig, int band)
>  {
>  	while (fa) {
>  		struct fown_struct * fown;
> @@ -719,22 +728,19 @@ void __kill_fasync(struct fasync_struct *fa, int sig, int band)
>  		   mechanism. */
>  		if (!(sig == SIGURG && fown->signum == 0))
>  			send_sigio(fown, fa->fa_fd, band);
> -		fa = fa->fa_next;
> +		fa = rcu_dereference(fa->fa_next);
>  	}
>  }
>  

Since rcu_read_lock() protects fasync_struct *fa for us, we can access
to @fa safely even fasync_remove_entry() is just called.

But this patch does not ensure 'fa->fa_file is not freed' nor
'fa->fa_fd is not released', so kill_fasync_rcu() may do wrong thing
if there is no other code ensure it.