netdev - Re: [PATCH bpf-next 12/17] nfp: remove rcu_read

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Date:   Fri, 11 Jun 2021 18:30:46 +0200
From:   Simon Horman <simon.horman@...ronome.com>
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>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        oss-drivers@...ronome.com, Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH bpf-next 12/17] nfp: remove rcu_read_lock() around XDP
 program invocation

+Jakub

On Wed, Jun 09, 2021 at 12:33:21PM +0200, Toke Høiland-Jørgensen wrote:
> The nfp 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.
> 
> While this is not actually an issue for the nfp driver because it doesn't
> support XDP_REDIRECT (and thus doesn't call xdp_do_flush()), the
> rcu_read_lock() is still unneeded. And 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: Simon Horman <simon.horman@...ronome.com>
> Cc: oss-drivers@...ronome.com
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>

Reviewed-by: Simon Horman <simon.horman@...ronome.com>

> ---
>  drivers/net/ethernet/netronome/nfp/nfp_net_common.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> index eeb30680b4dc..5dfa4799c34f 100644
> --- a/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> +++ b/drivers/net/ethernet/netronome/nfp/nfp_net_common.c
> @@ -1819,7 +1819,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
>  	struct xdp_buff xdp;
>  	int idx;
>  
> -	rcu_read_lock();
>  	xdp_prog = READ_ONCE(dp->xdp_prog);
>  	true_bufsz = xdp_prog ? PAGE_SIZE : dp->fl_bufsz;
>  	xdp_init_buff(&xdp, PAGE_SIZE - NFP_NET_RX_BUF_HEADROOM,
> @@ -2036,7 +2035,6 @@ static int nfp_net_rx(struct nfp_net_rx_ring *rx_ring, int budget)
>  			if (!nfp_net_xdp_complete(tx_ring))
>  				pkts_polled = budget;
>  	}
> -	rcu_read_unlock();
>  
>  	return pkts_polled;
>  }
> -- 
> 2.31.1
>