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

From: Oleg Nesterov <oleg@redhat.com>
To: Christian Brauner <christian.brauner@ubuntu.com>
Cc: linux-kernel@vger.kernel.org, libc-alpha@sourceware.org,
	alistair23@gmail.com, ebiederm@xmission.com, arnd@arndb.de,
	dalias@libc.org, torvalds@linux-foundation.org,
	adhemerval.zanella@linaro.org, fweimer@redhat.com,
	palmer@sifive.com, macro@wdc.com, zongbox@gmail.com,
	akpm@linux-foundation.org, viro@zeniv.linux.org.uk,
	hpa@zytor.com
Subject: Re: [PATCH v2 1/1] waitid: Add support for waiting for the current process group
Date: Wed, 14 Aug 2019 17:27:12 +0200	[thread overview]
Message-ID: <20190814152712.GE11595@redhat.com> (raw)
In-Reply-To: <20190814143545.tu6xfp2mxmnzwkx4@wittgenstein>

On 08/14, Christian Brauner wrote:
>
> 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. ;)

find_get_pid() actually tries to find a pid. The helper above does "find"
or "use current" depending on nr != 0.

> > 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);
> }

Who else can ever use it?

It saves 4 lines in kernel_waitid() but adds 7 lines outside, and you
need another ^] to see these lines if you try to understand what
PIDTYPE_PGID actually does. IOW, I think this helper will make waitid
less readable for no reason.

Christian, I try to never argue when it comes to cosmetic issues, and
in this case I won't insist too.

Oleg.