Re: [PATCH v16 02/16] cgroup/misc: Add per resource callbacks for CSS events

["Huang, Kai" <kai.huang@intel.com>]

On Tue, 2024-08-20 at 18:53 -0700, Haitao Huang wrote:
> +/**
> + * misc_cg_set_ops() - register resource specific operations.
> + * @type: Type of the misc res.
> + * @ops: Operations for the given type.
> + *
> + * The callbacks in @ops will not be invoked if the capacity of @type is 0.
> + *
> + * Context: Any context.
> + * Return:
> + * * %0 - Successfully registered the operations.
> + * * %-EINVAL - If @type is invalid, or the operations missing any required callbacks.
> + */
> +int misc_cg_set_ops(enum misc_res_type type, const struct misc_res_ops *ops)
> +{
> +	if (!valid_type(type))
> +		return -EINVAL;
> +
> +	if (!ops)
> +		return -EINVAL;
> +
> +	if (!ops->alloc) {
> +		pr_err("%s: alloc missing\n", __func__);
> +		return -EINVAL;
> +	}
> +
> +	if (!ops->free) {
> +		pr_err("%s: free missing\n", __func__);
> +		return -EINVAL;
> +	}
> +
> +	misc_res_ops[type] = ops;
> +	return 0;
> +}
> +EXPORT_SYMBOL_GPL(misc_cg_set_ops);

As mentioned in another reply, the export isn't mandatory for SGX cgroup
support due to SGX driver cannot be a module.

But I understand the intention is to treat this in a similar way to
misc_cg_set_capacity() etc which are currently exported.

I have no hard opinion whether to export this one.  But if we export this, I
don't quite like the fact that it doesn't accept the NULL ops to allow the
caller to unwind.

But I'll leave to cgroup maintainers.

> +
>  /**
>   * misc_cg_cancel_charge() - Cancel the charge from the misc cgroup.
>   * @type: Misc res type in misc cg to cancel the charge from.
> @@ -439,6 +477,36 @@ static struct cftype misc_cg_files[] = {
>  	{}
>  };
>  
> +static inline void _misc_cg_res_free(struct misc_cg *cg, enum misc_res_type last)
> +{
> +	enum misc_res_type i;
> +
> +	for (i = 0; i <= last; i++)
> +		if (misc_res_ops[i] && READ_ONCE(misc_res_capacity[i]))
> +			misc_res_ops[i]->free(cg);
> +}
> +
> +static inline int _misc_cg_res_alloc(struct misc_cg *cg)
> +{
> +	enum misc_res_type i;
> +	int ret;
> +
> +	for (i = 0; i < MISC_CG_RES_TYPES; i++) {
> +		WRITE_ONCE(cg->res[i].max, MAX_NUM);
> +		atomic64_set(&cg->res[i].usage, 0);
> +
> +		if (misc_res_ops[i] && READ_ONCE(misc_res_capacity[i])) {
> +			ret = misc_res_ops[i]->alloc(cg);
> +			if (ret) {
> +				_misc_cg_res_free(cg, i);
> +				return ret;
> +			}
> +		}
> +	}
> +
> +	return 0;
> +}
> +

I don't fully understand why kernel uses READ_ONCE()/WRITE_ONCE() to access
misc_res_capacity[i] and others like 'cg->res[i].max', but it looks weird they
are not used when accessing misc_res_ops[i]?