linux-kernel - Re: [PATCH v2 1/1] waitid: Add support for waiting for the current process group

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Date:   Wed, 14 Aug 2019 16:35:46 +0200
From:   Christian Brauner <christian.brauner@...ntu.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     linux-kernel@...r.kernel.org, libc-alpha@...rceware.org,
        alistair23@...il.com, ebiederm@...ssion.com, arnd@...db.de,
        dalias@...c.org, torvalds@...ux-foundation.org,
        adhemerval.zanella@...aro.org, fweimer@...hat.com,
        palmer@...ive.com, macro@....com, zongbox@...il.com,
        akpm@...ux-foundation.org, viro@...iv.linux.org.uk, hpa@...or.com
Subject: Re: [PATCH v2 1/1] waitid: Add support for waiting for the current
 process group

On Wed, Aug 14, 2019 at 04:19:57PM +0200, Oleg Nesterov wrote:
> On 08/14, Christian Brauner wrote:
> >
> > +static struct pid *find_get_pgrp(pid_t nr)
> > +{
> > +	struct pid *pid;
> > +
> > +	if (nr)
> > +		return find_get_pid(nr);
> > +
> > +	rcu_read_lock();
> > +	pid = get_pid(task_pgrp(current));
> > +	rcu_read_unlock();
> > +
> > +	return pid;
> > +}
> 
> I can't say I like this helper... even its name doesn't look good to me.

Well, naming scheme obviously stolen from find_get_pid(). Not sure if
that doesn't look good as well. ;)

> 
> I forgot that we already have get_task_pid() when I replied to the previous
> version... How about
> 
> 	case P_PGID:
> 
> 		if (upid)
> 			pid = find_get_pid(upid);
> 		else
> 			pid = get_task_pid(current, PIDTYPE_PGID);
> 
> ?

Hmyeah, that works but wouldn't it still be nicer to simply have:

static struct pid *get_pgrp(pid_t nr)
{
	if (nr)
		return find_get_pid(nr);

	return get_task_pid(current, PIDTYPE_PGID);
}

That allows us to have all cases equivalent with a single call to a
function without any if-elses.
Imho, this becomes especially nice when considering that P_PIDFD goes on
top of that:

	switch (which) {
	case P_ALL:
		type = PIDTYPE_MAX;
		break;
	case P_PID:
		type = PIDTYPE_PID;
		if (upid <= 0)
			return -EINVAL;

		pid = find_get_pid(upid);
		break;
	case P_PGID:
		type = PIDTYPE_PGID;
		if (upid < 0)
			return -EINVAL;

		pid = find_get_pgrp(upid);
		break;
	case P_PIDFD:
		type = PIDTYPE_PID;
		if (upid < 0)
			return -EINVAL;

		pid = pidfd_get_pid(upid);
		if (IS_ERR(pid))
			return PTR_ERR(pid);
		break;
	default:
		return -EINVAL;
	}

Christian