Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API

[Andrei Vagin]

On Thu, May 20, 2021 at 11:36:09AM -0700, Peter Oskolkov wrote:
> @@ -67,7 +137,75 @@ SYSCALL_DEFINE4(umcg_register_task, u32, api_version, u32, flags, u32, group_id,
>   */
>  SYSCALL_DEFINE1(umcg_unregister_task, u32, flags)
>  {
> -	return -ENOSYS;
> +	struct umcg_task_data *utd;
> +	int ret = -EINVAL;
> +
> +	rcu_read_lock();
> +	utd = rcu_dereference(current->umcg_task_data);
> +
> +	if (!utd || flags)
> +		goto out;
> +
> +	task_lock(current);
> +	rcu_assign_pointer(current->umcg_task_data, NULL);
> +	task_unlock(current);
> +
> +	ret = 0;
> +
> +out:
> +	rcu_read_unlock();
> +	if (!ret && utd) {
> +		synchronize_rcu();

synchronize_rcu is expensive. Do we really need to call it here? Can we
use kfree_rcu?

Where is task->umcg_task_data freed when a task is destroyed?

> +		kfree(utd);
> +	}
> +	return ret;
> +}
> +