netdev - Re: [PATCH net v2] net: Lock lower level devices when updating features

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Message-ID: <aBzYAzPtf_TlhT0n@mini-arch>
Date: Thu, 8 May 2025 09:12:51 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Cosmin Ratiu <cratiu@...dia.com>
Cc: netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
	Jakub Kicinski <kuba@...nel.org>,
	Eric Dumazet <edumazet@...gle.com>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Stanislav Fomichev <sdf@...ichev.me>,
	Saeed Mahameed <saeedm@...dia.com>,
	Tariq Toukan <tariqt@...dia.com>,
	Dragos Tatulea <dtatulea@...dia.com>
Subject: Re: [PATCH net v2] net: Lock lower level devices when updating
 features

On 05/08, Cosmin Ratiu wrote:
> __netdev_update_features() expects the netdevice to be ops-locked, but
> it gets called recursively on the lower level netdevices to sync their
> features, and nothing locks those.
> 
> This commit fixes that, with the assumption that it shouldn't be possible
> for both higher-level and lover-level netdevices to require the instance
> lock, because that would lead to lock dependency warnings.
> 
> Without this, playing with higher level (e.g. vxlan) netdevices on top
> of netdevices with instance locking enabled can run into issues:
> 
> WARNING: CPU: 59 PID: 206496 at ./include/net/netdev_lock.h:17 netif_napi_add_weight_locked+0x753/0xa60
> [...]
> Call Trace:
>  <TASK>
>  mlx5e_open_channel+0xc09/0x3740 [mlx5_core]
>  mlx5e_open_channels+0x1f0/0x770 [mlx5_core]
>  mlx5e_safe_switch_params+0x1b5/0x2e0 [mlx5_core]
>  set_feature_lro+0x1c2/0x330 [mlx5_core]
>  mlx5e_handle_feature+0xc8/0x140 [mlx5_core]
>  mlx5e_set_features+0x233/0x2e0 [mlx5_core]
>  __netdev_update_features+0x5be/0x1670
>  __netdev_update_features+0x71f/0x1670
>  dev_ethtool+0x21c5/0x4aa0
>  dev_ioctl+0x438/0xae0
>  sock_ioctl+0x2ba/0x690
>  __x64_sys_ioctl+0xa78/0x1700
>  do_syscall_64+0x6d/0x140
>  entry_SYSCALL_64_after_hwframe+0x4b/0x53
>  </TASK>
> 
> Fixes: 7e4d784f5810 ("net: hold netdev instance lock during rtnetlink operations")
> Signed-off-by: Cosmin Ratiu <cratiu@...dia.com>
> ---
>  net/core/dev.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 1be7cb73a602..4b5df59d6246 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -10454,7 +10454,9 @@ static void netdev_sync_lower_features(struct net_device *upper,
>  			netdev_dbg(upper, "Disabling feature %pNF on lower dev %s.\n",
>  				   &feature, lower->name);
>  			lower->wanted_features &= ~feature;
> +			netdev_lock_ops(lower);
>  			__netdev_update_features(lower);
> +			netdev_unlock_ops(lower);
>  
>  			if (unlikely(lower->features & feature))
>  				netdev_WARN(upper, "failed to disable %pNF on %s!\n",

Any reason not to cover the whole section under the if()? For example,
looking at netdev_features_change, most of its invocations are under the
lock, so keeping the lock around it might help with consistency (and
we can clarify it as such in Documentation/networking/netdevices.rst).
Plus, wanted_features is already sort of ops-protected (looking at
netif_disable_lro+dev_disable_lro).