linux-kernel - Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo

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Date:   Tue, 15 Oct 2019 16:43:57 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.com>
Cc:     linux-kernel@...r.kernel.org, Shuah Khan <shuah@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Michal Hocko <mhocko@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Christian Kellner <christian@...lner.me>,
        Roman Gushchin <guro@...com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Aleksa Sarai <cyphar@...har.com>,
        "Dmitry V. Levin" <ldv@...linux.org>,
        linux-kselftest@...r.kernel.org, Jann Horn <jannh@...gle.com>,
        linux-api@...r.kernel.org
Subject: Re: [PATCH 1/2] pidfd: verify task is alive when printing fdinfo

On 10/15, Christian Brauner wrote:
>
> +static inline bool task_alive(struct pid *pid)
> +{
> +	bool alive = true;
> +
> +	rcu_read_lock();
> +	if (!pid_task(pid, PIDTYPE_PID))
> +		alive = false;
> +	rcu_read_unlock();
> +
> +	return alive;
> +}

Well, the usage of rcu_read_lock/unlock looks confusing to me...

I mean, this helper does not need rcu lock at all. Except
rcu_dereference_check() will complain.

	static inline bool task_alive(struct pid *pid)
	{
		bool alive;

		/* shut up rcu_dereference_check() */
		rcu_lock_acquire(&rcu_lock_map);
		alive = !!pid_task(pid, PIDTYPE_PID));
		rcu_lock_release(&rcu_lock_map);

		return alive;
	}

looks more clear imo.

But in fact I'd suggest to simply use !hlist_empty(&pid->tasks[PIDTYPE_PID])
in pidfd_show_fdinfo() and do not add a new helper.

Oleg.