On 12/04, Frederic Weisbecker wrote:
>
> On Wed, Dec 04, 2013 at 02:49:17PM +0100, Oleg Nesterov wrote:
> >
> > Yes, perhaps we will need for_each_thread_continue(). I am not sure
> > yet. And note that, say, check_hung_uninterruptible_tasks() already
> > does _continue if fact, although it is still not clear to me if we
> > actually need this helper.
>
> So that's one of the possible users. _continue() can make sense if the
> reader can easily cope with missing a few threads from time to time, which
> is the case of the hung task detector.
Yes, but again it is not clear if we need the new helper.
For example. Note that you can simply do something like:
// p can't go away
rcu_read_lock();
for_each_thread(p, t) {
do_something(t);
if (need_to_sleep()) {
get_task_struct(t);
rcu_read_unlock();
schedule_timeout_interruptible(...);
rcu_read_lock();
put_task_struct();
if (!pid_alive(t))
break;
}
}
rcu_read_unlock();
If you rewrite this code using for_each_thread_continue (which is just
list_for_each_entry_continue_rcu) the code will look more complex.
> > Note also that _continue() can't be safely used lockless, unless
> > you verify pid_alive() or something similar.
>
> Hmm, due to concurrent list_del()?
>
> Right, tsk->thread_list.next could point to junk after a list_del(), say if
> the next
> entry has been freed.
Yes. The same problem which while_each_thread() currently has (I mean,
even ignoring the fact it is itself buggy).
Oleg.
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