linux-kernel - Re: [PATCH 1/2] creds: kill __task_cred()->task_is

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Message-ID: <5425.1316536050@redhat.com>
Date:	Tue, 20 Sep 2011 17:27:30 +0100
From:	David Howells <dhowells@...hat.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	dhowells@...hat.com, "Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"Serge E. Hallyn" <serge.hallyn@...onical.com>,
	lkml <linux-kernel@...r.kernel.org>, richard@....at,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 1/2] creds: kill __task_cred()->task_is_dead() check

Oleg Nesterov <oleg@...hat.com> wrote:

> From 8f92054e commit:
> 
>     Instead, add the following validation condition:
> 
>         task->exit_state >= 0
> 
>     to permit the access if the target task is dead and therefore unable to change
>     its own credentials.
> 
> OK, but afaics currently this can only help wait_task_zombie() which
> calls __task_cred() without rcu lock.
> 
> Remove this validation and change wait_task_zombie() to use task_uid()
> instead. This means we do rcu_read_lock() only to shut up the lockdep,
> but we already do the same in, say, wait_task_stopped().
> 
> Unfortunately, we can't kill task_is_dead() right now, it has already
> found the users in drivers/staging/, and I bet the usage is wrong.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

Acked-by: David Howells <dhowells@...hat.com>

It simplifies stuff in exchange for the most minor of slowdowns, so fair
enough.
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