netdev - Re: [PATCH bpf-next 05/17] ena: remove rcu_read

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Message-ID: <20210609135724.GB4397@paulmck-ThinkPad-P17-Gen-1>
Date:   Wed, 9 Jun 2021 06:57:24 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Toke Høiland-Jørgensen <toke@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        Martin KaFai Lau <kafai@...com>,
        Hangbin Liu <liuhangbin@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Guy Tzalik <gtzalik@...zon.com>,
        Saeed Bishara <saeedb@...zon.com>
Subject: Re: [PATCH bpf-next 05/17] ena: remove rcu_read_lock() around XDP
 program invocation

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@...zon.com>
> Cc: Saeed Bishara <saeedb@...zon.com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.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
>