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

Eric W. Biederman
2007-04-10T14:53:01Z

Ingo Molnar <mingo@elte.hu> writes:

> * Eric W. Biederman <ebiederm@xmission.com> wrote:
>
>> Ingo Molnar <mingo@elte.hu> writes:
>>
>> > no. Two _completely separate_ lists.
>> >
>> > i.e. a to-be-reaped task will still be on the main list _too_. The
>> > main list is for all the PID semantics rules. The reap-list is just
>> > for wait4() processing. The two would be completely separate.
>>
>> And what pray tell except for heuristics is the list of children used
>> for?
>
> 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.

Eric