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SubjectRe: init's children list is long and slows reaping children.

* Eric W. Biederman <ebiederm@xmission.com> wrote:

> > on a second thought: the p->children list is needed for the whole
> > child/parent task tree, which is needed for sys_getppid().
>
> Yes, something Oleg said made me realize that.
>
> As long as the reparent isn't to complex it isn't required that we
> have exactly one list .
>
> > The question is, does anything require us to reparent to within the
> > same thread group?
>
> I think my head is finally on straight about this question.
>
> Currently there is the silly linux specific parent death signal
> (pdeath_signal). Oleg's memory was a better than mine on this score.
>
> However there is no indication that the parent death signal being sent
> when a thread leader dies is actually correct, or even interesting. It
> probably should only be sent when getppid changes.
>
> So with pdeath_signal fixed that is nothing that requires us to
> reparent within the same thread group.
>
> I'm trying to remember what the story is now. There is a nasty race
> somewhere with reparenting, a threaded parent setting SIGCHLD to
> SIGIGN, and non-default signals that results in an zombie that no one
> can wait for and reap. It requires being reparented twice to trigger.
>
> Anyway it is a real mess and if we can remove the stupid multi-headed
> child lists things would become much simpler and the problem could not
> occur.
>
> Plus the code would become much simpler...
>
> utrace appears to have removed the ptrace_children list and the
> special cases that entailed.

so ... is anyone pursuing this? This would allow us to make sys_wait4()
faster and more scalable: no tasklist_lock bouncing for example.

Ingo
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