From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757493Ab0DXAqF (ORCPT ); Fri, 23 Apr 2010 20:46:05 -0400 Received: from e7.ny.us.ibm.com ([32.97.182.137]:46702 "EHLO e7.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757363Ab0DXAp7 (ORCPT ); Fri, 23 Apr 2010 20:45:59 -0400 Date: Fri, 23 Apr 2010 19:45:57 -0500 From: "Serge E. Hallyn" To: David Howells , "Paul E. McKenney" Cc: lkml Subject: [PATCH] Documentation/cred.txt: spinlock no longer implies rcu_read_lock Message-ID: <20100424004557.GA21297@us.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.20 (2009-06-14) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org So change the credentials documentation to make it clear that rcu read lock is required. Signed-off-by: Serge Hallyn --- 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