From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758593Ab0DAB30 (ORCPT ); Wed, 31 Mar 2010 21:29:26 -0400 Received: from e4.ny.us.ibm.com ([32.97.182.144]:46910 "EHLO e4.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758582Ab0DAB3X (ORCPT ); Wed, 31 Mar 2010 21:29:23 -0400 Date: Wed, 31 Mar 2010 18:29:19 -0700 From: "Paul E. McKenney" To: David Howells Cc: Eric Dumazet , 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] Message-ID: <20100401012919.GK2461@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <19192.1269889348@redhat.com> <23274.1269893706@redhat.com> <25276.1269901350@redhat.com> <26760.1269903543@redhat.com> <20100329232636.GT2569@linux.vnet.ibm.com> <2440.1269967151@redhat.com> <21972.1269993064@redhat.com> <10818.1270044273@redhat.com> <15371.1270057054@redhat.com> <19556.1270076008@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <19556.1270076008@redhat.com> 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 On Wed, Mar 31, 2010 at 11:53:28PM +0100, David Howells wrote: > Eric Dumazet wrote: > > > If you dont own a lock, and test a pointer, what guarantee do you have > > this pointer doesnt change right after you tested it ? > > There are five possibilities: > > (1) A pointer points to something when you check, and still points to the > same thing after you've gained the lock. > > (2) A pointer points to something when you check, and points to something > else after you've gained the lock. > > (3) A pointer points to something when you check, and is NULL after you've > gained the lock. > > (4) A pointer points to NULL when you check, and points to something after > you've gained the lock. > > (5) A pointer points to NULL when you check, and points to NULL after you've > gained the lock. > > However, what if you _know_ that the pointer can only ever be made non-NULL > during initialisation, and may even be left unset? That means possibility (4) > can never happen, and that possibility (5) can be detected by testing before > taking the lock. Now, what if (5) is a common occurrence? It might make > sense to make the test. > > And what matter if the pointer _does_ change after you test it. If it was > NULL before, it can only be NULL now - by the semantics defined for that > particular pointer. > > > If *something* protects the pointer from being changed, then how can be > > expressed this fact ? > > > > If nothing protects the pointer, why test it then, as result of test is > > unreliable ? > > I think you may be misunderstanding the purpose of rcu_dereference(). It is > to make sure the reading and dereferencing of the pointer are correctly > ordered with respect to the setting up of the pointed to record and the > changing of the pointer. > > There must be two memory accesses for the barrier implied to be of use. In > nfs_inode_return_delegation() there aren't two memory accesses to order, > therefore the barrier is pointless. > > > If NFS was using rcu_dereference(), it probably was for a reason, but if > > nobody can recall it, it was a wrong reason ? > > I think it is incorrectly used. Given that the rcu_dereference() in: > > if (rcu_dereference(nfsi->delegation) != NULL) { > spin_lock(&clp->cl_lock); > delegation = nfs_detach_delegation_locked(nfsi, NULL); > spin_unlock(&clp->cl_lock); > if (delegation != NULL) > nfs_do_return_delegation(inode, delegation, 0); > } And nfs_detach_delegation_locked() rechecks nfsi->delegation() under the lock, so this is a legitimate use. The pointer is not held constant, but any changes will be accounted for and handled correctly. So I would argue that the pointer value is in fact protected by the recheck-under-lock algorithm used here. Thanx, Paul > resolves to: > > _________p1 = nfsi->delegation; > smp_read_barrier_depends(); > if (_________p1) { > spin_lock(&clp->cl_lock); // implicit LOCK-class barrier > ==>nfs_detach_delegation_locked(nfsi, NULL); > [dereference nfsi->delegation] > ... > } > > do you actually need the smp_read_barrier_depends()? You _have_ a barrier in > the form of the spin_lock(). In fact, the spin_lock() is avowedly sufficient > to protect accesses to and dereferences of nfsi->delegation, which means that: > > static struct nfs_delegation *nfs_detach_delegation_locked(struct nfs_inode *nfsi, const nfs4_stateid *stateid) > { > struct nfs_delegation *delegation = rcu_dereference(nfsi->delegation); > ... > } > > has no need of the internal barrier provided by rcu_dereference() either. > > David