[Devel] Re: [PATCH][usercr]: Ghost tasks must be detached

[Louis Rilling]

On 10/02/11  9:54 -0800, Sukadev Bhattiprolu wrote:
> Louis Rilling [Louis.Rilling at kerlabs.com] wrote:
> | > I can reproduce a crash with 2.6.32 - where if container-init terminates
> | > before a detached child, we get a crash when the detached child calls
> | > proc_flush_mnt(). I suspected it was because do_wait_thread() skipped
> | > over detached tasks (in 2.6.32).
> | > 
> | > The same test case does not crash on 2.6.37 - which includes the above commit.
> | > The removes the check for detached tasks, my initial guess is that the above
> | > commit, may have contributed to _fixing_ the crash in 2.6.37.
> | 
> | Hm, I don't see how this commit changed things for detached tasks, unless ptrace
> | is involved. Detached tasks go atomically
> | from ->exit_state == 0 to ->exit_state == EXIT_DEAD in exit_notify(),
> | because tracehook_notify_death() returns DEATH_REAP for all not ptraced detached
> | tasks.
> | 
> | What do you think has changed precisely?
> 
> Well, one of the changes in the commit is this:
> 
> @@ -1551,14 +1554,9 @@ static int do_wait_thread(struct wait_opts *wo, struct task_struct *tsk)
>         struct task_struct *p;
> 
>         list_for_each_entry(p, &tsk->children, sibling) {
> -               /*
> -                * Do not consider detached threads.
> -                */
> -               if (!task_detached(p)) {
> -                       int ret = wait_consider_task(wo, 0, p);
> -                       if (ret)
> -                               return ret;
> -               }
> +               int ret = wait_consider_task(wo, 0, p);
> +               if (ret)
> +                       return ret;
>         }
> 
>         return 0;
> 
> ---
> If it was a detached task, do_wait_thread() skipped over it. In the C/R
> kernel we were setting the ->exit_signal to -1 for a "ghost" process. 
> I assumed that the container-init exited without waiting for the "ghost"
> and we were getting the crash in proc_flush_mnt() when the ghost exited.

The point is that wait_consider_task() skips detached tasks as soon as they are
not ptraced. So removing the check in do_wait_thread() should not have changed
the behavior. Am I missing something?

Thanks,

Louis

-- 
Dr Louis Rilling			Kerlabs
Skype: louis.rilling			Batiment Germanium
Phone: (+33|0) 6 80 89 08 23		80 avenue des Buttes de Coesmes
http://www.kerlabs.com/			35700 Rennes
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