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 17:27:12 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Christian Brauner <christian.brauner@...ntu.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 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.