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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
