linux-kernel - Re: [PATCH 1/3] posix-timers: don't switch to ->group_leader if ->it

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Date:	Mon, 4 Aug 2008 15:23:46 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	Roland McGrath <roland@...hat.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Mark McLoughlin <markmc@...hat.com>,
	Michael Kerrisk <mtk.manpages@...glemail.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] posix-timers: don't switch to ->group_leader if ->it_process dies

On 08/03, Roland McGrath wrote:
>
> The key thing that the old code does it to drop the ref to the old task as
> soon as it notices.

I thought about this too. But, again, this doesn't actually work. And it is
very easy to pin the dead task_struct. The user can just create the
SIGEV_THREAD_ID timer and kill the sub-thread (->it_process). Without
calling timer_settime() the timer can't notice the death.

(just in case, please note that the next patch doesn't make things worse,
 when ->group_leader dies the timer should be destroyed soon).

> It may be worthwhile to do that in the case where
> send_sigqueue fails.

I think it is better to convert the posix timers to use "struct pid*" instead
of "struct task_struct*". This change looks more or less straightforward, and
it should fix the problem.

Oleg.

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