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Subject: Re: [PATCH] UDPCP Communication Protocol
From: Stefani Seibold <stefani@seibold.net>
To: Eric Dumazet <eric.dumazet@gmail.com>
Cc: linux-kernel@vger.kernel.org, akpm@linux-foundation.org,
        davem@davemloft.net, netdev@vger.kernel.org
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Date: Sat, 01 Jan 2011 22:28:45 +0100
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Am Freitag, den 31.12.2010, 13:00 +0100 schrieb Eric Dumazet:
> 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.
> 

I still revamped the whole /proc/net/udpcp thing and hope it is now race
free.