On Sat, Mar 14, 2015 at 08:18:36PM +0100, Oleg Nesterov wrote: > On 03/14, Josh Triplett wrote: > > > > On Sat, Mar 14, 2015 at 11:38:29AM -0700, Thiago Macieira wrote: > > > On Saturday 14 March 2015 15:32:35 Oleg Nesterov wrote: > > > > It is not clear to me what do_wait() should do with ->autoreap child, even > > > > ignoring ptrace. > > > > > > > > Just suppose that real_parent has a single "autoreap" child. Should > > > > wait(NULL) hanf then? > > > > > > It should ignore the child that is set to autoreap. wait(NULL) should return - > > > ECHILD, indicating there are no children waiting to be reaped. > > > > Right. And I don't think the current code does this. I think we need > > to change wait_consider_task to early-return for ->autoreap just as it > > does for task_state == EXIT_DEAD. > > No. This EXIT_DEAD is absolutely different. And this is another indication > that you might use it wrongly ;) Is there any information somewhere on how this state machine of doom is *supposed* to work? :) Why would "p->task_state == EXIT_DEAD" mean something different in wait_consider_task? > What we actually want is BUG_ON(task_state == EXIT_DEAD) here. We do not > want the EXIT_DEAD tasks in ->children/ptraced lists. These EXIT_DEAD tasks > complicate the exit/wait/reparent paths. Pulling the EXIT_DEAD tasks out of those lists completely does sound like a good simplification. However, that doesn't seem to be the current expectation in wait_consider_task, which just returns if p->task_state == EXIT_DEAD to skip considering that task. And an autoreaping task isn't necessarily dead yet; it just shouldn't be waited on. > However, currently this is TODO. The main problem is the locking in > wait_task_zombie(), we can set EXIT_DEAD and remove the task from list > under read_lock(). That appears to be only reachable for zombies, which an autoreaping task should never become. > And please see another email from me. So far I disagree that wait(NULL) > should return ECHILD unconditionally. At least unless this is discussed > separately. I'll respond in that separate thread, but one issue there: waiting for any child process cannot safely return an autoreaping child process, because that would introduce a race condition. The PID the parent gets back can disappear at any time, so there's nothing useful the parent can do with it. - Josh Triplett -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
