netdev - Re: [PATCH] netfilter: remove an atomic bit operation

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Date:	Sat, 20 Nov 2010 17:50:04 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	tim.gardner@...onical.com
Cc:	Changli Gao <xiaosuo@...il.com>, Patrick McHardy <kaber@...sh.net>,
	"David S. Miller" <davem@...emloft.net>,
	netfilter-devel@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH] netfilter: remove an atomic bit operation

Le samedi 20 novembre 2010 à 09:03 -0700, Tim Gardner a écrit :
> On 11/20/2010 12:55 AM, Changli Gao wrote:
> > As this ct won't be seen by the others, we don't need to set the
> > IPS_CONFIRMED_BIT in atomic way.
> >
> > Signed-off-by: Changli Gao<xiaosuo@...il.com>
> > ---
> >   net/netfilter/nf_conntrack_core.c |    2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> > diff --git a/net/netfilter/nf_conntrack_core.c b/net/netfilter/nf_conntrack_core.c
> > index 27a5ea6..c708248 100644
> > --- a/net/netfilter/nf_conntrack_core.c
> > +++ b/net/netfilter/nf_conntrack_core.c
> > @@ -486,7 +486,7 @@ __nf_conntrack_confirm(struct sk_buff *skb)
> >   	ct->timeout.expires += jiffies;
> >   	add_timer(&ct->timeout);
> >   	atomic_inc(&ct->ct_general.use);
> > -	set_bit(IPS_CONFIRMED_BIT,&ct->status);
> > +	ct->status |= IPS_CONFIRMED_BIT;
> >
> >   	/* Since the lookup is lockless, hash insertion must be done after
> >   	 * starting the timer and setting the CONFIRMED bit. The RCU barriers
> > --

> NAK, set_bit() takes a bit number, not a mask. That is, assuming you can 
> get away with a non-atomic operation on this field. I'll defer to 
> Patrick on that.
> 
> I think you have to use IPS_CONFIRMED instead, e.g.,
> 
> 	ct->status |= IPS_CONFIRMED;
> 

Or

	__set_bit(IPS_CONFIRMED_BIT, &ct->status);


set_bit() is atomic, while __set_bit() is not



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