From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932924Ab0DGRPL (ORCPT ); Wed, 7 Apr 2010 13:15:11 -0400 Received: from mx1.redhat.com ([209.132.183.28]:14384 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932825Ab0DGRPI (ORCPT ); Wed, 7 Apr 2010 13:15:08 -0400 Organization: Red Hat UK Ltd. Registered Address: Red Hat UK Ltd, Amberley Place, 107-111 Peascod Street, Windsor, Berkshire, SI4 1TE, United Kingdom. Registered in England and Wales under Company Registration No. 3798903 From: David Howells Subject: [PATCH] RCU: Better explain the condition parameter of rcu_dereference_check() To: paulmck@linux.vnet.ibm.com, eric.dumazet@gmail.com Cc: linux-kernel@vger.kernel.org, David Howells Date: Wed, 07 Apr 2010 18:15:00 +0100 Message-ID: <20100407171500.27710.88994.stgit@warthog.procyon.org.uk> User-Agent: StGIT/0.14.3 MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Better explain the condition parameter of rcu_dereference_check() that describes the conditions under which the dereference is permitted to take place. This condition is only checked under lockdep proving. Signed-off-by: David Howells --- include/linux/rcupdate.h | 28 +++++++++++++++++++++++----- 1 files changed, 23 insertions(+), 5 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 872a98e..acf9958 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -195,12 +195,30 @@ static inline int rcu_read_lock_sched_held(void) /** * rcu_dereference_check - rcu_dereference with debug checking + * @p: The pointer to read, prior to dereferencing + * @c: The conditions under which the dereference will take place * - * Do an rcu_dereference(), but check that the context is correct. - * For example, rcu_dereference_check(gp, rcu_read_lock_held()) to - * ensure that the rcu_dereference_check() executes within an RCU - * read-side critical section. It is also possible to check for - * locks being held, for example, by using lockdep_is_held(). + * Do an rcu_dereference(), but check that the conditions under which the + * dereference will take place are correct. Typically the conditions indicate + * the various locking conditions that should be held at that point. The check + * should return true if the conditions are satisfied. + * + * For example: + * + * bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() || + * lockdep_is_held(&foo->lock)); + * + * could be used to indicate to lockdep that foo->bar may only be dereferenced + * if either the RCU read lock is held, or that the lock required to replace + * the bar struct at foo->bar is held. + * + * Note that the list of conditions may also include indications of when a lock + * need not be held, for example during initialisation or destruction of the + * target struct: + * + * bar = rcu_dereference_check(foo->bar, rcu_read_lock_held() || + * lockdep_is_held(&foo->lock), + * atomic_read(&foo->usage) == 0); */ #define rcu_dereference_check(p, c) \ ({ \