linux-kernel - Re: broken do_each_pid

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Date:	Mon, 15 Dec 2008 11:50:33 +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 12/14, 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.

> Actually, I don't understand why the compiler complains, and I never
> saw a warning myself.

Because the `if' is not reachable :). (And it's not compiler which complains
here.)

>> After investigating this code usage all around, it's broken on many places
>> this or similar way.
>>
>> For do_each_pid_thread(), even this code snippet from fs/ioprio.c is broken
>> due to double do {} while expansion:
>> do_each_pid_thread(pgrp, PIDTYPE_PGID, p) {
>>   ret = set_task_ioprio(p, ioprio);
>>   if (ret)
>>     break;
>> } while_each_pid_thread(pgrp, PIDTYPE_PGID, p);
> 
> 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.
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