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

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Date:	Tue, 20 Sep 2011 17:40:15 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.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

On 09/20, Paul E. McKenney wrote:
>
> On Tue, Sep 20, 2011 at 04:39:42PM +0200, Oleg Nesterov wrote:
> >
> > --- 3.1/include/linux/cred.h~1_kill_task_is_dead	2011-09-20 16:28:22.000000000 +0200
> > +++ 3.1/include/linux/cred.h	2011-09-20 16:28:47.000000000 +0200
> > @@ -284,8 +284,7 @@ static inline void put_cred(const struct
> >  #define __task_cred(task)						\
> >  	({								\
> >  		const struct task_struct *__t = (task);			\
> > -		rcu_dereference_check(__t->real_cred,			\
> > -				      task_is_dead(__t));		\
> > +		rcu_dereference_check(__t->real_cred, 0);		\
>
> The "0" above will make lockdep-RCU complain unconditionally.  My guess
> is that you want rcu_dereference_raw().

Hmm. I hope you are wrong this time ;)

rcu_dereference_check() checks rcu_read_lock_held(). IOW, with this
change __task_cred() always requires rcu_read_lock(), and this is
what the patch wants.

The next one adds " || (task == current" to the rcu_read_lock_held()
check above.

Oleg.

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