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Date:   Fri, 21 May 2021 12:06:07 -0700
From:   Andrei Vagin <avagin@gmail.com>
To:     Peter Oskolkov <posk@google.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        Paul Turner <pjt@google.com>, Ben Segall <bsegall@google.com>,
        Peter Oskolkov <posk@posk.io>,
        Joel Fernandes <joel@joelfernandes.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andrei Vagin <avagin@google.com>,
        Jim Newsome <jnewsome@torproject.org>
Subject: Re: [RFC PATCH v0.1 4/9] sched/umcg: implement core UMCG API
Message-ID: <YKgEn/TBsTyhk0m1@gmail.com>
References: <20210520183614.1227046-1-posk@google.com>
 <20210520183614.1227046-5-posk@google.com>
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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;
> +}
> +