Re: [PATCH net-next 11/12] net/mlx5: HWS, Free unused action STE tables

[Michal Kubiak <michal.kubiak@intel.com>]

On Tue, Apr 08, 2025 at 05:00:55PM +0300, Tariq Toukan wrote:
> From: Vlad Dogaru <vdogaru@nvidia.com>
> 
> Periodically check for unused action STE tables and free their
> associated resources. In order to do this safely, add a per-queue lock
> to synchronize the garbage collect work with regular operations on
> steering rules.
> 
> Signed-off-by: Vlad Dogaru <vdogaru@nvidia.com>
> Reviewed-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
> Reviewed-by: Mark Bloch <mbloch@nvidia.com>
> Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
> ---
>  .../mlx5/core/steering/hws/action_ste_pool.c  | 88 ++++++++++++++++++-
>  .../mlx5/core/steering/hws/action_ste_pool.h  | 11 +++
>  .../mellanox/mlx5/core/steering/hws/context.h |  1 +
>  3 files changed, 96 insertions(+), 4 deletions(-)

[...]

> +
> +static void hws_action_ste_pool_cleanup(struct work_struct *work)
> +{
> +	enum mlx5hws_pool_optimize opt;
> +	struct mlx5hws_context *ctx;
> +	LIST_HEAD(cleanup);
> +	int i;
> +
> +	ctx = container_of(work, struct mlx5hws_context,
> +			   action_ste_cleanup.work);
> +
> +	for (i = 0; i < ctx->queues; i++) {
> +		struct mlx5hws_action_ste_pool *p = &ctx->action_ste_pool[i];
> +
> +		mutex_lock(&p->lock);
> +		for (opt = MLX5HWS_POOL_OPTIMIZE_NONE;
> +		     opt < MLX5HWS_POOL_OPTIMIZE_MAX; opt++)
> +			hws_action_ste_pool_element_collect_stale(
> +				&p->elems[opt], &cleanup);
> +		mutex_unlock(&p->lock);
> +	}

As I understand, in the loop above all unused items are being collected
to remove them at the end of the function, using `hws_action_ste_table_cleanup_list()`.

I noticed that only the collecting of elements is protected with the mutex.
So I have a question: is it possible that in a very short period of time
(between `mutex_unlock()` and `hws_action_ste_table_cleanup_list()` calls),
the cleanup list can somehow be invalidated (by changing the STE state
in another thread)?

> +
> +	hws_action_ste_table_cleanup_list(&cleanup);
> +
> +	schedule_delayed_work(&ctx->action_ste_cleanup,
> +			      secs_to_jiffies(
> +				  MLX5HWS_ACTION_STE_POOL_CLEANUP_SECONDS));
> +}
> +

[...]

Thanks,
Michal