Re: [PATCH v2 1/4] introduce for_each_thread() to replace the buggy while_each_thread()

[Frederic Weisbecker <fweisbec@gmail.com>]

On Wed, Dec 04, 2013 at 02:49:17PM +0100, Oleg Nesterov wrote:
> On 12/04, Frederic Weisbecker wrote:
> >
> > On Wed, Dec 04, 2013 at 02:04:09PM +0100, Oleg Nesterov wrote:
> >
> > > For example, do/while_each_thread() always
> > > sees at least one task, while for_each_thread() can do nothing if
> > > the whole thread group has died.
> >
> > Would it be safe to have for_each_thread_continue() instead?
> 
> Yes, and no.
> 
> 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.

> 
> But no, _continue() can't help if the whole thread group has died,
> we simply can not continue.

Right, but if the whole group has died, the list is empty anyway. I mean
pure rcu walking requires the user to tolerate the miss of some concurrent
updates anyway.

> 
> 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.

> 
> And,
> 
> > Yeah if the conversion needs careful audit, it makes sense to switch incrementally.
> 
> Yes. For example the case above. If someone does
> 
> 	do
> 		do_something(t);
> 	while_each_thread(g, t);
> 
> we should check that it can tolerate the case when do_something()
> won't be called at all, or ensure that this is not possible.

Right!

Thanks.