linux-kernel - Re: broken do_each_pid

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Message-ID: <20081215110238.GA15606@redhat.com>
Date:	Mon, 15 Dec 2008 12:02:38 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Jiri Slaby <jirislaby@...il.com>
Cc:	kenchen@...gle.com,
	Linux kernel mailing list <linux-kernel@...r.kernel.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: broken do_each_pid_{thread,task}

On 12/15, Jiri Slaby wrote:
>
> Oleg Nesterov napsal(a):
> > On 12/14, Jiri Slaby wrote:
> >> I'm getting
> >> `if (type == PIDTYPE_PID)' is unreachable
> >> warning from kernel/exit.c. The preprocessed code looks like:
> >> do {
> >>          struct hlist_node *pos___;
> >>          if (pgrp != ((void *)0))
> >>                  for (LIST ITERATION) {
> >>                          {
> >>                           if (!((p->state & 4) != 0))
> >>                            continue;
> >>                           retval = 1;
> >>                           break;
> >>                          }
> >>                          if (PIDTYPE_PGID == PIDTYPE_PID)
> >>                                  break;
> >>                  }
> >> } while (0);
> >> and it's obviously wrong.
> >
> > Why do you think it is wrong? This break stops the "hlist_for_each"
> > loop, not the enclosing "do while".
>
> The `continue' matters here (and also in other do_each_pid_task cases).
> Sorry for not mentioning it explicitly.

Still can't understand... OK, I think we misundersood each other.
Do you agree that the code is technically correct? Or I missed
something?

"continue" looks fine to me too, it is also for the inner loop.

> > Actually, I don't understand why the compiler complains, and I never
> > saw a warning myself.
>
> Because the `if' is not reachable :).

Yes, I see it is not reachable, but I don't understand why this
deserves a warning ;)

Look, "if (PIDTYPE_PGID == PIDTYPE_PID)" is not possible too, should
the compiler (or whatever) complain?
	
> (And it's not compiler which complains
> here.)

Ah, OK, thanks. Just curious, and who does?

> > Yes, this is obviously not what was intended. But afaics, this is
> > the only place which should be fixed?
>
> Actually yes. And add a big warning to the macros or whatever to not get
> into it later again.

Agreed.

Oleg.

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