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

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Date:	Tue, 10 Apr 2007 08:51:43 -0600
From:	ebiederm@...ssion.com (Eric W. Biederman)
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Oleg Nesterov <oleg@...sign.ru>, Robin Holt <holt@....com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Chris Snook <csnook@...hat.com>, linux-kernel@...r.kernel.org,
	Jack Steiner <steiner@...ricas.sgi.com>
Subject: Re: init's children list is long and slows reaping children.

Ingo Molnar <mingo@...e.hu> writes:

> * Eric W. Biederman <ebiederm@...ssion.com> wrote:
>
>> Ingo Molnar <mingo@...e.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

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