From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH net-next 3/3] tap: XDP support
Date: Fri, 28 Jul 2017 06:46:40 +0300
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Cc: Jakub Kicinski <kubakici@wp.pl>, davem@davemloft.net,
        netdev@vger.kernel.org, linux-kernel@vger.kernel.org
To: Jason Wang <jasowang@redhat.com>
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On Fri, Jul 28, 2017 at 11:28:54AM +0800, Jason Wang wrote:
> > > +	old_prog = rtnl_dereference(tun->xdp_prog);
> > > +	if (old_prog)
> > > +		bpf_prog_put(old_prog);
> > > +	rcu_assign_pointer(tun->xdp_prog, prog);
> > Is this OK?  Could this lead to the program getting freed and then
> > datapath accessing a stale pointer?  I mean in the scenario where the
> > process gets pre-empted between the bpf_prog_put() and
> > rcu_assign_pointer()?
> 
> Will call bpf_prog_put() after rcu_assign_pointer().

I suspect you need to sync RCU or something before that.