linux-kernel - Re: [PATCH RFC 1/2] Add polling support to pidfd

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Date:   Tue, 16 Apr 2019 14:04:31 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org, luto@...capital.net,
        rostedt@...dmis.org, dancol@...gle.com, christian@...uner.io,
        jannh@...gle.com, surenb@...gle.com, torvalds@...ux-foundation.org,
        Alexey Dobriyan <adobriyan@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Andrei Vagin <avagin@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        Michal Hocko <mhocko@...e.com>, Nadav Amit <namit@...are.com>,
        Serge Hallyn <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Taehee Yoo <ap420073@...il.com>, Tejun Heo <tj@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH RFC 1/2] Add polling support to pidfd

On 04/11, Joel Fernandes (Google) wrote:
>
> +static unsigned int proc_tgid_base_poll(struct file *file, struct poll_table_struct *pts)
> +{
> +	int poll_flags = 0;
> +	struct task_struct *task;
> +	struct pid *pid;
> +
> +	task = get_proc_task(file->f_path.dentry->d_inode);
> +
> +	WARN_ON_ONCE(task && !thread_group_leader(task));
> +
> +	/*
> +	 * tasklist_lock must be held because to avoid racing with
> +	 * changes in exit_state and wake up. Basically to avoid:
> +	 *
> +	 * P0: read exit_state = 0
> +	 * P1: write exit_state = EXIT_DEAD
> +	 * P1: Do a wake up - wq is empty, so do nothing
> +	 * P0: Queue for polling - wait forever.
> +	 */
> +	read_lock(&tasklist_lock);
> +	if (!task)
> +		poll_flags = POLLIN | POLLRDNORM | POLLERR;
> +	else if (task->exit_state == EXIT_DEAD)
> +		poll_flags = POLLIN | POLLRDNORM;
> +	else if (task->exit_state == EXIT_ZOMBIE && thread_group_empty(task))
> +		poll_flags = POLLIN | POLLRDNORM;
> +
> +	if (!poll_flags) {
> +		pid = proc_pid(file->f_path.dentry->d_inode);
> +		poll_wait(file, &pid->wait_pidfd, pts);
> +	}

can't understand...

Could you explain when it should return POLLIN? When the whole process exits?

Then all you need is

	!task || task->exit_state && thread_group_empty(task)

Please do not use EXIT_DEAD/EXIT_ZOMBIE. And ->wait_pidfd should probably
live in task->signal_struct.

Oleg.