Re: [PATCH net] net/mlx5: Don't zero user_count when destroying FDB tables

[Simon Horman]

On Sun, Oct 26, 2025 at 10:20:19PM +0200, Tariq Toukan wrote:
> From: Cosmin Ratiu <cratiu@xxxxxxxxxx>
> 
> esw->user_count tracks how many TC rules are added on an esw via
> mlx5e_configure_flower -> mlx5_esw_get -> atomic64_inc(&esw->user_count)
> 
> esw.user_count was unconditionally set to 0 in
> esw_destroy_legacy_fdb_table and esw_destroy_offloads_fdb_tables.
> 
> These two together can lead to the following sequence of events:
> 1. echo 1 > /sys/class/net/eth2/device/sriov_numvfs
>   - mlx5_core_sriov_configure -...-> esw_create_legacy_table ->
>     atomic64_set(&esw->user_count, 0)
> 2. tc qdisc add dev eth2 ingress && \
>    tc filter replace dev eth2 pref 1 protocol ip chain 0 ingress \
>        handle 1 flower action ct nat zone 64000 pipe
>   - mlx5e_configure_flower -> mlx5_esw_get ->
>     atomic64_inc(&esw->user_count)
> 3. echo 0 > /sys/class/net/eth2/device/sriov_numvfs
>   - mlx5_core_sriov_configure -..-> esw_destroy_legacy_fdb_table
>     -> atomic64_set(&esw->user_count, 0)
> 4. devlink dev eswitch set pci/0000:08:00.0 mode switchdev
>   - mlx5_devlink_eswitch_mode_set -> mlx5_esw_try_lock ->
>     atomic64_read(&esw->user_count) == 0
>   - then proceed to a WARN_ON in:
>   esw_offloads_start -> mlx5_eswitch_enable_locke -> esw_offloads_enable
>   -> mlx5_esw_offloads_rep_load -> mlx5e_vport_rep_load ->
>   mlx5e_netdev_change_profile -> mlx5e_detach_netdev ->
>   mlx5e_cleanup_nic_rx -> mlx5e_tc_nic_cleanup ->
>   mlx5e_mod_hdr_tbl_destroy
> 
> Fix this by not clearing out the user_count when destroying FDB tables,
> so that the check in mlx5_esw_try_lock can prevent the mode change when
> there are TC rules configured, as originally intended.
> 
> Fixes: 2318b8bb94a3 ("net/mlx5: E-switch, Destroy legacy fdb table when needed")
> Signed-off-by: Cosmin Ratiu <cratiu@xxxxxxxxxx>
> Reviewed-by: Dragos Tatulea <dtatulea@xxxxxxxxxx>
> Signed-off-by: Tariq Toukan <tariqt@xxxxxxxxxx>

Reviewed-by: Simon Horman <horms@xxxxxxxxxx>