Re: [Intel-wired-lan] [PATCH iwl-next] ice: allow hot-swapping XDP programs

[Maciej Fijalkowski <maciej.fijalkowski@intel.com>]

On Tue, Jun 13, 2023 at 05:15:15PM +0200, Alexander Lobakin wrote:
> From: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> Date: Tue, 13 Jun 2023 17:10:05 +0200
> 
> > Currently ice driver's .ndo_bpf callback brings the interface down and
> > up independently of the presence of XDP resources. This is only needed
> > when either these resources have to be configured or removed. It means
> > that if one is switching XDP programs on-the-fly with running traffic,
> > packets will be dropped.
> > 
> > To avoid this, compare early on ice_xdp_setup_prog() state of incoming
> > bpf_prog pointer vs the bpf_prog pointer that is already assigned to
> > VSI. Do the swap in case VSI has bpf_prog and incoming one are non-NULL.
> > 
> > Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
> 
> [0] :D
> 
> But if be serious, are you sure you won't have any pointer tears /
> partial reads/writes without such RCU protection as added in the
> linked commit ?

Since we removed rcu sections from driver sides and given an assumption
that local_bh_{dis,en}able() pair serves this purpose now i believe this
is safe. Are you aware of:

https://lore.kernel.org/bpf/20210624160609.292325-1-toke@redhat.com/

?

> 
> > ---
> >  drivers/net/ethernet/intel/ice/ice_main.c | 12 +++++-------
> >  1 file changed, 5 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/net/ethernet/intel/ice/ice_main.c b/drivers/net/ethernet/intel/ice/ice_main.c
> > index a1f7c8edc22f..8940ee505811 100644
> > --- a/drivers/net/ethernet/intel/ice/ice_main.c
> > +++ b/drivers/net/ethernet/intel/ice/ice_main.c
> > @@ -2922,6 +2922,11 @@ ice_xdp_setup_prog(struct ice_vsi *vsi, struct bpf_prog *prog,
> >  					   "MTU is too large for linear frames and XDP prog does not support frags");
> >  			return -EOPNOTSUPP;
> >  		}
> > +
> > +	/* hot swap progs and avoid toggling link */
> > +	if (ice_is_xdp_ena_vsi(vsi) == !!prog) {
> > +		ice_vsi_assign_bpf_prog(vsi, prog);
> > +		return 0;
> >  	}
> >  
> >  	/* need to stop netdev while setting up the program for Rx rings */
> > @@ -2956,13 +2961,6 @@ ice_xdp_setup_prog(struct ice_vsi *vsi, struct bpf_prog *prog,
> >  		xdp_ring_err = ice_realloc_zc_buf(vsi, false);
> >  		if (xdp_ring_err)
> >  			NL_SET_ERR_MSG_MOD(extack, "Freeing XDP Rx resources failed");
> > -	} else {
> > -		/* safe to call even when prog == vsi->xdp_prog as
> > -		 * dev_xdp_install in net/core/dev.c incremented prog's
> > -		 * refcount so corresponding bpf_prog_put won't cause
> > -		 * underflow
> > -		 */
> > -		ice_vsi_assign_bpf_prog(vsi, prog);
> >  	}
> >  
> >  	if (if_running)
> 
> [0]
> https://github.com/alobakin/linux/commit/5645adb0943dabeadfb9f6d00202c78fb9594fbe
> 
> Thanks,
> Olek