netdev - Re: [PATCH] sfc: adjust efx->xdp_tx_queue_count with the real number of initialized queues

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Date:   Thu, 29 Apr 2021 15:22:06 +0100
From:   Edward Cree <ecree.xilinx@...il.com>
To:     Ignat Korchagin <ignat@...udflare.com>, habetsm.xilinx@...il.com,
        davem@...emloft.net, kuba@...nel.org, netdev@...r.kernel.org
Cc:     kernel-team@...udflare.com, stable@...r.kernel.org
Subject: Re: [PATCH] sfc: adjust efx->xdp_tx_queue_count with the real number
 of initialized queues

On 27/04/2021 22:09, Ignat Korchagin wrote:
> efx->xdp_tx_queue_count is initially initialized to num_possible_cpus() and is
> later used to allocate and traverse efx->xdp_tx_queues lookup array. However,
> we may end up not initializing all the array slots with real queues during
> probing. This results, for example, in a NULL pointer dereference, when running
> "# ethtool -S <iface>", similar to below
...
> diff --git a/drivers/net/ethernet/sfc/efx_channels.c b/drivers/net/ethernet/sfc/efx_channels.c
> index 1bfeee283ea9..a3ca406a3561 100644
> --- a/drivers/net/ethernet/sfc/efx_channels.c
> +++ b/drivers/net/ethernet/sfc/efx_channels.c
> @@ -914,6 +914,8 @@ int efx_set_channels(struct efx_nic *efx)
>  			}
>  		}
>  	}
> +	if (xdp_queue_number)
Wait, why is this guard condition needed?
What happens if we had nonzero efx->xdp_tx_queue_count initially, but we end up
 with no TXQs available for XDP at all (so xdp_queue_number == 0)?

-ed
> +		efx->xdp_tx_queue_count = xdp_queue_number;
>  
>  	rc = netif_set_real_num_tx_queues(efx->net_dev, efx->n_tx_channels);
>  	if (rc)
>