From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753741Ab0C2UPT (ORCPT ); Mon, 29 Mar 2010 16:15:19 -0400 Received: from mx1.redhat.com ([209.132.183.28]:62091 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753676Ab0C2UPP (ORCPT ); Mon, 29 Mar 2010 16:15:15 -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 In-Reply-To: <20100329192159.GM2569@linux.vnet.ibm.com> References: <20100329192159.GM2569@linux.vnet.ibm.com> <20100319022527.GC2894@linux.vnet.ibm.com> <20100318133302.29754.1584.stgit@warthog.procyon.org.uk> <19192.1269889348@redhat.com> To: paulmck@linux.vnet.ibm.com Cc: dhowells@redhat.com, Trond.Myklebust@netapp.com, linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() [ver #2] Date: Mon, 29 Mar 2010 21:15:06 +0100 Message-ID: <23274.1269893706@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Paul E. McKenney wrote: > > > if (rcu_dereference_check(nfsi->delegation, > > > lockdep_is_held(&clp->cl_lock)) != NULL) { > > > > If clp->cl_lock protects this pointer, why the need for > > rcu_dereference_check() at all? The check is redundant since the line > > above gets the very lock we're checking for. > > Because Arnd Bergmann is working on a set of patches that makes sparse > complain if you access an RCU-protected pointer directly, without using > some flavor of rcu_dereference(). > > So your approach would work for the moment, but would need another > change, probably in the 2.6.35 timeframe. My objection to using rcu_dereference_check() here is that it's a dynamic check: the compiler emits code to do it, since the lock/unlock status of what the pointer points to cannot be determined easily at compiler time - and then the barrier is interpolated anyway unnecessarily. David