linux-kernel - Re: broken do_each_pid

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Date:	Mon, 15 Dec 2008 12:33:01 +0100
From:	Jiri Slaby <jirislaby@...il.com>
To:	Oleg Nesterov <oleg@...hat.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}

Oleg Nesterov napsal(a):
> 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.

But it doesn't jump to the `if' (this is what I would expect from the
`continue' here), but to the third statement of the `for'.

Maybe better to ask, is the test expected to be fired after *each*
invocation of the body?

>>> 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?

Correct, in this particular case (and I checked that also other users which
uses `continue' inside the loop don't pass PIDTYPE_PID).

>> (And it's not compiler which complains
>> here.)
> 
> Ah, OK, thanks. Just curious, and who does?

A static analyzer. Stay tuned, we will announce it later, it's in the state
of development :).
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