Re: [PATCH] simplify/fix first_tid()

[Oleg Nesterov <oleg@tv-sign.ru>]

"Eric W. Biederman" wrote:
> 
> Oleg Nesterov <oleg@tv-sign.ru> writes:
> 
> > first_tid:
> >
> >       /* If nr exceeds the number of threads there is nothing todo */
> >       if (nr) {
> >               if (nr >= get_nr_threads(leader))
> >                       goto done;
> >       }
> >
> > This is not reliable: sub-threads can exit after this check, so the
> > 'for' loop below can overlap and proc_task_readdir() can return an
> > already filldir'ed dirents.
> >
> >       for (; pos && pid_alive(pos); pos = next_thread(pos)) {
> >               if (--nr > 0)
> >                       continue;
> >
> > Off-by-one error, will return 'leader' when nr == 1.
> >
> > This patch tries to fix these problems and simplify the code.
> 
> This is better however if I read this code correctly.  It modifies
> the code so the last time user space goes trough this loop
> with nr > nr_threads.  Then we will walk the entire threads
> list to achieve nothing.

This can happen only if the thread we stopped at has exited, and
some other threads have exited too, so that nr >= ->signal->count.

I think it's not worth optimizing this rare and anyway slow path.
However, you are the code author, I'll send a trivial patch which
restores this optimization if you don't change you mind.

> So we really still need the nr_threads test in there so we don't
> traverse the list twice everytime through readdir.

How so? We don't do it twice?

Oleg.