linux-kernel - Re: [PATCH 2/2] creds: __task_cred(current) doesn't need rcu_read_lock

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Date:	Tue, 20 Sep 2011 10:07:27 -0500
From:	Serge Hallyn <serge.hallyn@...onical.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	David Howells <dhowells@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	lkml <linux-kernel@...r.kernel.org>, richard@....at,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH 2/2] creds: __task_cred(current) doesn't need
 rcu_read_lock_held()

Quoting Oleg Nesterov (oleg@...hat.com):
> Change __task_cred(task) to accept "task == current" without
> rcu_read_lock_held(). This is what current_cred() currently does,
> and with this change __task_cred() becomes more flexible/usable.
> 
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

So to check whether I've got this straight, the original __task_cred()
was allowing rcu read lock to not be held for a non-running task, but
required rcu read lock if task was running?  With these two patches,
rcu read lock will not be needed if task == current?

If so, then that sounds good to me, and an unconditional rcu_read_lock()
at wait_task_zombie() seems better than at send_signal()...

> ---
> 
>  include/linux/cred.h |    5 ++---
>  1 file changed, 2 insertions(+), 3 deletions(-)
> 
> --- 3.1/include/linux/cred.h~2_task_cred_ck_current	2011-09-20 16:28:47.000000000 +0200
> +++ 3.1/include/linux/cred.h	2011-09-20 16:33:12.000000000 +0200
> @@ -268,8 +268,7 @@ static inline void put_cred(const struct
>   * Access the subjective credentials of the current task.  RCU-safe,
>   * since nobody else can modify it.
>   */
> -#define current_cred() \
> -	rcu_dereference_protected(current->cred, 1)
> +#define current_cred()	__task_cred(current)
>  
>  /**
>   * __task_cred - Access a task's objective credentials
> @@ -284,7 +283,7 @@ static inline void put_cred(const struct
>  #define __task_cred(task)						\
>  	({								\
>  		const struct task_struct *__t = (task);			\
> -		rcu_dereference_check(__t->real_cred, 0);		\
> +		rcu_dereference_check(__t->real_cred, (__t == current));\
>  	})
>  
>  /**
> 
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