netdev - Re: [PATCH] UDPCP Communication Protocol

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Date:	Fri, 31 Dec 2010 13:00:05 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	stefani@...bold.net
Cc:	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	davem@...emloft.net, netdev@...r.kernel.org
Subject: Re: [PATCH] UDPCP Communication Protocol

Le vendredi 31 décembre 2010 à 12:25 +0100, Eric Dumazet a écrit :
> Le vendredi 31 décembre 2010 à 10:29 +0100, stefani@...bold.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.




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