Re: broken do_each_pid_{thread,task}

[Oleg Nesterov <oleg@redhat.com>]

On 12/15, Jiri Slaby wrote:
>
> 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'.

Yes, but this doesn't matter,

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

Yes, but we need this only when type == PIDTYPE_PID, so the code
is correct.

> > 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).

Yes.

Don't get me wrong, I do agree (let me repeat again) this check is
absolutely ugly and may cause the problems.

As I said, it fixes the minor and only theoretical problem. Perhaps we
can move this check to the code  which does do_each_pid_task(PIDTYPE_PID).
Or better yet, just introduce for_each_pid_task() (see another email).

> >> (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 :).

Great, thanks ;)

Oleg.