Re: [PATCH] UDPCP Communication Protocol

[Eric Dumazet <eric.dumazet@gmail.com>]

Le vendredi 31 décembre 2010 à 12:25 +0100, Eric Dumazet a écrit :
> Le vendredi 31 décembre 2010 à 10:29 +0100, stefani@seibold.net a
> écrit :
> > +		if (!list_empty(&usk->destlist)) {
> > +			state->sk = (struct sock *)usk;
> > +			state->dest = list_first_entry(&usk->destlist,
> > +					struct udpcp_dest, list);
> > +			sock_hold(state->sk);
> > +
> > +			if (atomic_read(&state->sk->sk_refcnt) != 1) {
> > +				spin_unlock_irqrestore(&spinlock, flags);
> > +				return state;
> > +			}
> > +			atomic_dec(&state->sk->sk_refcnt);
> > +		}
> > +
> 
> I am trying to understand what you are doing here.
> 
> It seems racy to me.
> 
> Apparently, what you want is to take a reference only if actual
> sk_refcnt is not zero.
> 
> I suggest using atomic_inc_notzero(&state->sk->sk_refcnt) to avoid the
> race in atomic_dec().
> 
> 

Before you ask why its racy, this is because UDP sockets are RCU
protected, and RCU lookups depend on sk_refcnt being zero or not.

Doing an sk_refcnt increment/decrement opens a race window for the
concurrent lookups.