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

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Date:	Thu, 12 Apr 2007 00:19:57 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Bill Davidsen <davidsen@....com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Ingo Molnar <mingo@...e.hu>, 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.

On 04/11, Bill Davidsen wrote:
>
> Oleg Nesterov wrote:
> >On 04/10, Eric W. Biederman wrote:
> >
> >>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.
> >
> >reparent_thread:
> >
> >	...
> >
> >        /* If we'd notified the old parent about this child's death,
> >         * also notify the new parent.
> >         */
> >        if (!traced && p->exit_state == EXIT_ZOMBIE &&
> >            p->exit_signal != -1 && thread_group_empty(p))
> >                do_notify_parent(p, p->exit_signal);
> >
> >We notified /sbin/init. If it ignores SIGCHLD, we should release the task.
> >We don't do this.
> >
> >The best fix I believe is to cleanup the 
> >forget_original_parent/reparent_thread
> >interaction and factor out this "exit_state == EXIT_ZOMBIE && exit_signal 
> >== -1"
> >checks.
> >
> As long as the original parent is preserved for getppid(). There are 
> programs out there which communicate between the parent and child with 
> signals, and if the original parent dies, it undesirable to have the 
> child getppid() and start sending signals to a program not expecting 
> them. Invites undefined behavior.

Sorry, can't understand.

If p->exit_signal == -1 after do_notify_parent() above, the task is completely
dead. Nobody can release it, we should do this (if EXIT_ZOMBIE). At this point
"p" was already re-parented, but this (and getppid) doesn't matter at all.

OK. Most likely you meant something else. Could you clarify?

Oleg.

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