From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933143Ab0CPSLO (ORCPT ); Tue, 16 Mar 2010 14:11:14 -0400 Received: from e2.ny.us.ibm.com ([32.97.182.142]:36784 "EHLO e2.ny.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756636Ab0CPSLN (ORCPT ); Tue, 16 Mar 2010 14:11:13 -0400 Date: Tue, 16 Mar 2010 11:10:53 -0700 From: "Paul E. McKenney" To: Trond Myklebust Cc: David Howells , linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] NFS: Fix RCU warnings in nfs_inode_return_delegation_noreclaim() Message-ID: <20100316181053.GO6709@linux.vnet.ibm.com> Reply-To: paulmck@linux.vnet.ibm.com References: <20100316115130.25642.17789.stgit@warthog.procyon.org.uk> <20100316171738.GN6709@linux.vnet.ibm.com> <1268760954.3098.17.camel@localhost.localdomain> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1268760954.3098.17.camel@localhost.localdomain> User-Agent: Mutt/1.5.15+20070412 (2007-04-11) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Mar 16, 2010 at 01:35:54PM -0400, Trond Myklebust wrote: > On Tue, 2010-03-16 at 10:17 -0700, Paul E. McKenney wrote: > > On Tue, Mar 16, 2010 at 11:51:30AM +0000, David Howells wrote: > > > diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c > > > index 2563beb..a77c735 100644 > > > --- a/fs/nfs/delegation.c > > > +++ b/fs/nfs/delegation.c > > > @@ -37,8 +37,10 @@ static void nfs_free_delegation(struct nfs_delegation *delegation) > > > { > > > struct rpc_cred *cred; > > > > > > + rcu_read_lock(); > > > cred = rcu_dereference(delegation->cred); > > > rcu_assign_pointer(delegation->cred, NULL); > > > > The lock is probably held here, in which case something like the > > following would work well without needing the artificial rcu_read_lock() > > and rcu_read_unlock(): > > No. The lock is not held here. At this point, the delegation has been > detached from the inode that pointed to it, and so we can free up its > contents. OK. Is there some reference counter or pointer that can be checked to verify that this data structure really is in a state that prevents RCU readers from finding it? > We still need the call_rcu() to free up the allocated memory in order to > ensure that some process doing lockless traversal of the > clp->cl_delegations list doesn't crash. OK. > > cred = rcu_dereference_check(delegation->cred, > > lockdep_is_held(&delegation->lock)); > > > > > + rcu_read_unlock(); > > > call_rcu(&delegation->rcu, nfs_free_delegation_callback); > > > if (cred) > > > put_rpccred(cred); > > > @@ -212,10 +214,12 @@ int nfs_inode_set_delegation(struct inode *inode, struct rpc_cred *cred, struct > > > spin_lock_init(&delegation->lock); > > > > > > spin_lock(&clp->cl_lock); > > > + rcu_read_lock(); > > > if (rcu_dereference(nfsi->delegation) != NULL) { > > > > Same here, though I am not sure whether clp->cl_lock or something in > > nfs_inode should be used. > > Yes. As I indicated to David in another email, the clp->cl_lock protects > us here, so it looks as if your suggestion above would be perfect. Sounds good! Thanx, Paul > > > if (memcmp(&delegation->stateid, &nfsi->delegation->stateid, > > > sizeof(delegation->stateid)) == 0 && > > > delegation->type == nfsi->delegation->type) { > > > + rcu_read_unlock(); > > > goto out; > > > } > > > /* > > > @@ -228,6 +232,7 @@ int nfs_inode_set_delegation(struct inode *inode, struct rpc_cred *cred, struct > > > if (delegation->type <= nfsi->delegation->type) { > > > freeme = delegation; > > > delegation = NULL; > > > + rcu_read_lock(); > > > goto out; > > > } > > > freeme = nfs_detach_delegation_locked(nfsi, NULL); >