Re: [PATCH net v3] bonding: fix slave_cnt leak on XDP error paths

[Nikolay Aleksandrov <razor@blackwall.org>]

On 07/09/2026 10:07, Hangbin Liu wrote:
> From: Hangbin Liu <liuhangbin@kylinos.cn>
> 
> When bond_enslave() succeeds up to the XDP setup stage, slave_cnt is
> already incremented. If XDP setup subsequently fails, the error paths
> jump directly to err_sysfs_del, bypassing the slave_cnt decrement.
> 
> This causes slave_cnt to drift upward on each failed enslaving attempt,
> which would lead to unbalanced traffic distribution with round-robin
> mode.
> 
> Fix it by moving the slave_cnt increase after XDP setup.
> 
> Fixes: 9e2ee5c7e7c3 ("net, bonding: Add XDP support to the bonding driver")
> Signed-off-by: Hangbin Liu <liuhangbin@kylinos.cn>
> ---
> Changes in v3:
> - move the slave_cnt increasement before bond_update_slave_arr() (selftest)
> - run all bonding selftests on debug kernel to make sure no regression (Jakub)
> - Link to v2: https://lore.kernel.org/r/20260903-bond_slave_cnt-v2-1-02e27304ca36@kylinos.cn
> 
> Changes in v2:
> - move the slave_cnt increasement after XDP setup (Nikolay Aleksandrov)
> - balance-xor mode is not affected, not mention it (Nikolay Aleksandrov)
> - Link to v1: https://lore.kernel.org/r/20260902-bond_slave_cnt-v1-1-36e95bf4a6ff@kylinos.cn
> ---
>   drivers/net/bonding/bond_main.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/net/bonding/bond_main.c b/drivers/net/bonding/bond_main.c
> index a9bff7663eec..1514a8ba0888 100644
> --- a/drivers/net/bonding/bond_main.c
> +++ b/drivers/net/bonding/bond_main.c
> @@ -2284,7 +2284,6 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
>   		}
>   	}
>   
> -	WRITE_ONCE(bond->slave_cnt, bond->slave_cnt + 1);
>   	netdev_compute_master_upper_features(bond->dev, true);
>   	bond_set_carrier(bond);
>   
> @@ -2332,6 +2331,9 @@ int bond_enslave(struct net_device *bond_dev, struct net_device *slave_dev,
>   			bpf_prog_inc(bond->xdp_prog);
>   	}
>   
> +	/* Increase the slave count before rebuilding the slave arrays. */
> +	WRITE_ONCE(bond->slave_cnt, bond->slave_cnt + 1);
> +
>   	/* broadcast mode uses the all_slaves to loop through slaves. */
>   	if (bond_mode_can_use_xmit_hash(bond) ||
>   	    BOND_MODE(bond) == BOND_MODE_BROADCAST)
> 
> ---
> base-commit: 38b6be101006d3e7af972999f45d4f1e8250587a
> change-id: 20260807-bond_slave_cnt-88e78c5f0ab9
> 
> Best regards,

I think the comment is redundant, of course we should change slave_cnt before it
is used by anything that needs it.

Anyway, the patch looks good:
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>