netdev - Re: [PATCH net-next 3/3] tap: XDP support

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Message-ID: <20170728064603-mutt-send-email-mst@kernel.org>
Date:   Fri, 28 Jul 2017 06:46:40 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Jakub Kicinski <kubakici@...pl>, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 3/3] tap: XDP support

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.