Re: [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP program invocation

["Paul E. McKenney" <paulmck@kernel.org>]

On Wed, Jun 09, 2021 at 12:33:14PM +0200, Toke Høiland-Jørgensen wrote:
> The ena 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.

It might be worth adding a comment, perhaps where the rcu_read_lock()
used to be, stating what the protection is.  Maybe something like this?

	/*
	 * This code is invoked within a single NAPI poll cycle
	 * and thus under local_bh_disable(), which provides the
	 * needed RCU protection.
	 */

							Thanx, Paul

> Cc: Guy Tzalik <gtzalik@amazon.com>
> Cc: Saeed Bishara <saeedb@amazon.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
> ---
>  drivers/net/ethernet/amazon/ena/ena_netdev.c | 3 ---
>  1 file changed, 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/amazon/ena/ena_netdev.c b/drivers/net/ethernet/amazon/ena/ena_netdev.c
> index 881f88754bf6..a4378b14af4c 100644
> --- a/drivers/net/ethernet/amazon/ena/ena_netdev.c
> +++ b/drivers/net/ethernet/amazon/ena/ena_netdev.c
> @@ -385,7 +385,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)
>  	u64 *xdp_stat;
>  	int qid;
>  
> -	rcu_read_lock();
>  	xdp_prog = READ_ONCE(rx_ring->xdp_bpf_prog);
>  
>  	if (!xdp_prog)
> @@ -443,8 +442,6 @@ static int ena_xdp_execute(struct ena_ring *rx_ring, struct xdp_buff *xdp)
>  
>  	ena_increase_stat(xdp_stat, 1, &rx_ring->syncp);
>  out:
> -	rcu_read_unlock();
> -
>  	return verdict;
>  }
>  
> -- 
> 2.31.1
>