netdev - RE: [PATCH bpf-next v2 07/16] freescale: remove rcu_read

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Date:   Wed, 16 Jun 2021 14:58:36 +0000
From:   Camelia Alexandra Groza <camelia.groza@....com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     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>,
        Jakub Kicinski <kuba@...nel.org>,
        Madalin Bucur <madalin.bucur@....com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Ioana Ciocoi Radulescu <ruxandra.radulescu@....com>
Subject: RE: [PATCH bpf-next v2 07/16] freescale: remove rcu_read_lock()
 around XDP program invocation

> -----Original Message-----
> From: Toke Høiland-Jørgensen <toke@...hat.com>
> Sent: Tuesday, June 15, 2021 17:55
> To: bpf@...r.kernel.org; netdev@...r.kernel.org
> Cc: 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>;
> Jakub Kicinski <kuba@...nel.org>; Toke Høiland-Jørgensen
> <toke@...hat.com>; Madalin Bucur <madalin.bucur@....com>; Ioana
> Ciornei <ioana.ciornei@....com>; Ioana Ciocoi Radulescu
> <ruxandra.radulescu@....com>
> Subject: [PATCH bpf-next v2 07/16] freescale: remove rcu_read_lock()
> around XDP program invocation
> 
> The dpaa and dpaa2 drivers have 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: Madalin Bucur <madalin.bucur@....com>
> Cc: Ioana Ciornei <ioana.ciornei@....com>
> Cc: Ioana Radulescu <ruxandra.radulescu@....com>
> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
> ---

Reviewed-by: Camelia Groza <camelia.groza@....com>

Thanks!

>  drivers/net/ethernet/freescale/dpaa/dpaa_eth.c   | 11 ++++-------
>  drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c |  6 +++---
>  2 files changed, 7 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> index 177c020bf34a..98fdcbde687a 100644
> --- a/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> +++ b/drivers/net/ethernet/freescale/dpaa/dpaa_eth.c
> @@ -2558,13 +2558,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv,
> struct qm_fd *fd, void *vaddr,
>  	u32 xdp_act;
>  	int err;
> 
> -	rcu_read_lock();
> -
>  	xdp_prog = READ_ONCE(priv->xdp_prog);
> -	if (!xdp_prog) {
> -		rcu_read_unlock();
> +	if (!xdp_prog)
>  		return XDP_PASS;
> -	}
> 
>  	xdp_init_buff(&xdp, DPAA_BP_RAW_SIZE -
> DPAA_TX_PRIV_DATA_SIZE,
>  		      &dpaa_fq->xdp_rxq);
> @@ -2585,6 +2581,9 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv,
> struct qm_fd *fd, void *vaddr,
>  	}
>  #endif
> 
> +	/* This code is invoked within a single NAPI poll cycle and thus under
> +	 * local_bh_disable(), which provides the needed RCU protection.
> +	 */
>  	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
> 
>  	/* Update the length and the offset of the FD */
> @@ -2638,8 +2637,6 @@ static u32 dpaa_run_xdp(struct dpaa_priv *priv,
> struct qm_fd *fd, void *vaddr,
>  		break;
>  	}
> 
> -	rcu_read_unlock();
> -
>  	return xdp_act;
>  }
> 
> diff --git a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> index 8433aa730c42..964d85c9e37d 100644
> --- a/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> +++ b/drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c
> @@ -352,8 +352,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv
> *priv,
>  	u32 xdp_act = XDP_PASS;
>  	int err, offset;
> 
> -	rcu_read_lock();
> -
>  	xdp_prog = READ_ONCE(ch->xdp.prog);
>  	if (!xdp_prog)
>  		goto out;
> @@ -363,6 +361,9 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv
> *priv,
>  	xdp_prepare_buff(&xdp, vaddr + offset, XDP_PACKET_HEADROOM,
>  			 dpaa2_fd_get_len(fd), false);
> 
> +	/* This code is invoked within a single NAPI poll cycle and thus under
> +	 * local_bh_disable(), which provides the needed RCU protection.
> +	 */
>  	xdp_act = bpf_prog_run_xdp(xdp_prog, &xdp);
> 
>  	/* xdp.data pointer may have changed */
> @@ -414,7 +415,6 @@ static u32 dpaa2_eth_run_xdp(struct dpaa2_eth_priv
> *priv,
> 
>  	ch->xdp.res |= xdp_act;
>  out:
> -	rcu_read_unlock();
>  	return xdp_act;
>  }
> 
> --
> 2.31.1