Re: [PATCH bpf-next 14/17] sfc: remove rcu_read_lock() around XDP program invocation

[Edward Cree <ecree.xilinx@gmail.com>]

On 09/06/2021 11:33, Toke Høiland-Jørgensen wrote:
> The sfc driver has rcu_read_lock()/rcu_read_unlock() pairs around XDP
> program invocations. However, the actual lifetime of the objects referred
> by the XDP program invocation is longer, all the way through to the call to
> xdp_do_flush(), making the scope of the rcu_read_lock() too small. This
> turns out to be harmless because it all happens in a single NAPI poll
> cycle (and thus under local_bh_disable()), but it makes the rcu_read_lock()
> misleading.
> 
> Rather than extend the scope of the rcu_read_lock(), just get rid of it
> entirely. With the addition of RCU annotations to the XDP_REDIRECT map
> types that take bh execution into account, lockdep even understands this to
> be safe, so there's really no reason to keep it around.
> 
> Cc: Edward Cree <ecree.xilinx@gmail.com>
> Cc: Martin Habets <habetsm.xilinx@gmail.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
Acked-by: Edward Cree <ecree.xilinx@gmail.com>

>  drivers/net/ethernet/sfc/rx.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/sfc/rx.c b/drivers/net/ethernet/sfc/rx.c
> index 17b8119c48e5..606750938b89 100644
> --- a/drivers/net/ethernet/sfc/rx.c
> +++ b/drivers/net/ethernet/sfc/rx.c
> @@ -260,18 +260,14 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
>  	s16 offset;
>  	int err;
>  
> -	rcu_read_lock();
> -	xdp_prog = rcu_dereference(efx->xdp_prog);
> -	if (!xdp_prog) {
> -		rcu_read_unlock();
> +	xdp_prog = rcu_dereference_bh(efx->xdp_prog);
> +	if (!xdp_prog)
>  		return true;
> -	}
>  
>  	rx_queue = efx_channel_get_rx_queue(channel);
>  
>  	if (unlikely(channel->rx_pkt_n_frags > 1)) {
>  		/* We can't do XDP on fragmented packets - drop. */
> -		rcu_read_unlock();
>  		efx_free_rx_buffers(rx_queue, rx_buf,
>  				    channel->rx_pkt_n_frags);
>  		if (net_ratelimit())
> @@ -296,7 +292,6 @@ static bool efx_do_xdp(struct efx_nic *efx, struct efx_channel *channel,
>  			 rx_buf->len, false);
>  
>  	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
> -	rcu_read_unlock();
>  
>  	offset = (u8 *)xdp.data - *ehp;
>  
>