linux-kernel - Re: [PATCH 2/2] taskstats: add_del_listener() should ignore !valid listener's

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Date:	Wed, 20 Jul 2011 23:46:03 +0400
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Balbir Singh <bsingharora@...il.com>,
	Jerome Marchand <jmarchan@...hat.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] taskstats: add_del_listener() should ignore !valid
 listener's

On Wed, Jul 20, 2011 at 21:00 +0200, Oleg Nesterov wrote:
> When send_cpu_listeners() finds the orphaned listener it marks
> it as !valid and drops listeners->sem. Before it takes this sem
> for wrinting, s->pid can be reused and add_del_listener() can
> wrongly try to re-use this entry.
> 
> Change add_del_listener() to check ->valid = T.

I think the current logic is wrong.  If s was claimed is invalid, it
then points to a dead task, it is unknown when it died.  As the same pid
can be reused by a new process BEFORE add_del_listener(), it is unknown
whether ->pid points to the actual owner task.

Rather than optimizing the wrong algorithm it's better to change the
processes keeping way.

Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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