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From: Roland McGrath <roland@redhat.com>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, Ingo Molnar <mingo@elte.hu>,
       Ratan Nalumasu <rnalumasu@gmail.com>,
       Vitaly Mayatskikh <vmayatsk@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [RFC,PATCH 2/2] change __wake_up_parent() to use filtered
	wakeup
In-Reply-To: Oleg Nesterov's message of  Wednesday, 24 June 2009 18:11:11 +0200 <20090624161111.GA27890@redhat.com>
References: <20090622170437.GA4906@redhat.com>
	<20090624091316.73D0F4059B@magilla.sf.frob.com>
	<20090624152143.GB23848@redhat.com>
	<20090624185701.AA74C4059B@magilla.sf.frob.com>
	<20090624161111.GA27890@redhat.com>
Message-Id: <20090624194239.A29174059B@magilla.sf.frob.com>
Date: Wed, 24 Jun 2009 12:42:39 -0700 (PDT)
Sender: linux-kernel-owner@vger.kernel.org
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> > Agreed.  The not-right case I had in mind was do_notify_parent_cldstop
> > where maybe it could be ->real_parent of the untraced group_leader.
> 
> Hmm. Could you explain?
> 
> I feel I missed something, but can't understand.

Sorry, it wasn't very clear.  (I guess I should have said "traced group
leader", though that would not have been much more obvious.)

do_notify_parent_cldstop:

	if (task_ptrace(tsk))
		parent = tsk->parent;
	else {
		tsk = tsk->group_leader;
		parent = tsk->real_parent;
	}
	...
	__wake_up_parent(tsk, parent);

In the "else" case, parent is not necessarily tsk->parent.  That is, if
an untraced thread calls do_notify_parent_cldstop() but its group_leader
is ptrace_reparented().  Then waitee->group_leader->real_parent is who
gets the wakeup, but __wake_up_parent->child_wait_wakeup would check
only waiter == waitee->group_leader->parent.


Thanks,
Roland
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