linux-kernel - Re: [PATCH] Documentation/cred.txt: spinlock no longer implies rcu_read

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Message-ID: <20100424012430.GC2589@linux.vnet.ibm.com>
Date:	Fri, 23 Apr 2010 18:24:30 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Serge E. Hallyn" <serue@...ibm.com>
Cc:	David Howells <dhowells@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] Documentation/cred.txt: spinlock no longer implies
 rcu_read_lock

On Fri, Apr 23, 2010 at 07:45:57PM -0500, Serge E. Hallyn wrote:
> So change the credentials documentation to make it clear that rcu
> read lock is required.

Very good!

Reviewed-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>

> Signed-off-by: Serge Hallyn <serue@...ibm.com>
> ---
>  Documentation/credentials.txt |    8 ++------
>  1 files changed, 2 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/credentials.txt b/Documentation/credentials.txt
> index df03169..98a1e9e 100644
> --- a/Documentation/credentials.txt
> +++ b/Documentation/credentials.txt
> @@ -408,9 +408,6 @@ This should be used inside the RCU read lock, as in the following example:
>  		...
>  	}
> 
> -A function need not get RCU read lock to use __task_cred() if it is holding a
> -spinlock at the time as this implicitly holds the RCU read lock.
> -
>  Should it be necessary to hold another task's credentials for a long period of
>  time, and possibly to sleep whilst doing so, then the caller should get a
>  reference on them using:
> @@ -426,14 +423,13 @@ credentials, hiding the RCU magic from the caller:
>  	uid_t task_uid(task)		Task's real UID
>  	uid_t task_euid(task)		Task's effective UID
> 
> -If the caller is holding a spinlock or the RCU read lock at the time anyway,
> -then:
> +If the caller is holding the RCU read lock at the time anyway, then:
> 
>  	__task_cred(task)->uid
>  	__task_cred(task)->euid
> 
>  should be used instead.  Similarly, if multiple aspects of a task's credentials
> -need to be accessed, RCU read lock or a spinlock should be used, __task_cred()
> +need to be accessed, RCU read lock should be used, __task_cred()
>  called, the result stored in a temporary pointer and then the credential
>  aspects called from that before dropping the lock.  This prevents the
>  potentially expensive RCU magic from being invoked multiple times.
> -- 
> 1.6.3.3
> 
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