Subject: Re: [PATCH net v2] ixgbe: Panic during XDP_TX with > 64 CPUs

From: Maciej Fijalkowski <[email-masked]>
Date: Wed, 1 Feb 2023 19:50:08 +0100

On Mon, Jan 30, 2023 at 11:38:15PM -0800, John Hickey wrote:
> In commit 'ixgbe: let the xdpdrv work with more than 64 cpus'
> (4fe815850bdc), support was added to allow XDP programs to run on systems
> with more than 64 CPUs by locking the XDP TX rings and indexing them
> using cpu % 64 (IXGBE_MAX_XDP_QS).
> 
> Upon trying this out patch via the Intel 5.18.6 out of tree driver
> on a system with more than 64 cores, the kernel paniced with an
> array-index-out-of-bounds at the return in ixgbe_determine_xdp_ring in
> ixgbe.h, which means ixgbe_determine_xdp_q_idx was just returning the
> cpu instead of cpu % IXGBE_MAX_XDP_QS.

I'd like to ask you to include the splat you got in the commit message.

> 
> I think this is how it happens:
> 
> Upon loading the first XDP program on a system with more than 64 CPUs,
> ixgbe_xdp_locking_key is incremented in ixgbe_xdp_setup.  However,
> immediately after this, the rings are reconfigured by ixgbe_setup_tc.
> ixgbe_setup_tc calls ixgbe_clear_interrupt_scheme which calls
> ixgbe_free_q_vectors which calls ixgbe_free_q_vector in a loop.
> ixgbe_free_q_vector decrements ixgbe_xdp_locking_key once per call if
> it is non-zero.  Commenting out the decrement in ixgbe_free_q_vector
> stopped my system from panicing.
> 
> I suspect to make the original patch work, I would need to load an XDP
> program and then replace it in order to get ixgbe_xdp_locking_key back
> above 0 since ixgbe_setup_tc is only called when transitioning between
> XDP and non-XDP ring configurations, while ixgbe_xdp_locking_key is
> incremented every time ixgbe_xdp_setup is called.
> 
> Also, ixgbe_setup_tc can be called via ethtool --set-channels, so this
> becomes another path to decrement ixgbe_xdp_locking_key to 0 on systems
> with greater than 64 CPUs.
> 
> For this patch, I have changed static_branch_inc to static_branch_enable
> in ixgbe_setup_xdp.  We aren't counting references and I don't see any
> reason to turn it off, since all the locking appears to be in the XDP_TX
> path, which isn't run if a XDP program isn't loaded.
> 
> Fixes: 4fe815850bdc ("ixgbe: let the xdpdrv work with more than 64 cpus")
> Signed-off-by: John Hickey <[email-masked]>
> ---
> v1 -> v2:
> 	Added Fixes and net tag.  No code changes.
> ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c  | 3 ---
>  drivers/net/ethernet/intel/ixgbe/ixgbe_main.c | 2 +-
>  2 files changed, 1 insertion(+), 4 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> index f8156fe4b1dc..0ee943db3dc9 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_lib.c
> @@ -1035,9 +1035,6 @@ static void ixgbe_free_q_vector(struct ixgbe_adapter *adapter, int v_idx)
>  	adapter->q_vector[v_idx] = NULL;
>  	__netif_napi_del(&q_vector->napi);
>  
> -	if (static_key_enabled(&ixgbe_xdp_locking_key))
> -		static_branch_dec(&ixgbe_xdp_locking_key);

Yeah calling this per each qvector is *very* unbalanced approach whereas
you bump it single time when loading xdp prog.

> -
>  	/*
>  	 * after a call to __netif_napi_del() napi may still be used and
>  	 * ixgbe_get_stats64() might access the rings on this vector,
> diff --git a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> index ab8370c413f3..cd2fb72c67be 100644
> --- a/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> +++ b/drivers/net/ethernet/intel/ixgbe/ixgbe_main.c
> @@ -10283,7 +10283,7 @@ static int ixgbe_xdp_setup(struct net_device *dev, struct bpf_prog *prog)
>  	if (nr_cpu_ids > IXGBE_MAX_XDP_QS * 2)
>  		return -ENOMEM;
>  	else if (nr_cpu_ids > IXGBE_MAX_XDP_QS)
> -		static_branch_inc(&ixgbe_xdp_locking_key);
> +		static_branch_enable(&ixgbe_xdp_locking_key);

Now that you removed static_branch_dec you probably need a counter part
(static_branch_disable) at appriopriate place.

>  
>  	old_prog = xchg(&adapter->xdp_prog, prog);
>  	need_reset = (!!prog != !!old_prog);
> -- 
> 2.37.2
>