From: Dave Kleikamp Subject: Re: [EXT4 set 4][PATCH 1/5] i_version:64 bit inode version Date: Wed, 11 Jul 2007 09:28:06 -0500 Message-ID: <1184164086.12154.15.camel@kleikamp.austin.ibm.com> References: <1183275424.4010.126.camel@localhost.localdomain> <20070710163038.ceb2ae94.akpm@linux-foundation.org> <1184105380.3759.65.camel@localhost.localdomain> <20070710182237.e2f88bf3.akpm@linux-foundation.org> <18068.19667.942363.686858@notabene.brown> <18068.25879.745638.343290@notabene.brown> Mime-Version: 1.0 Content-Type: text/plain Content-Transfer-Encoding: 7bit Cc: cmm@us.ibm.com, Andrew Morton , linux-fsdevel@vger.kernel.org, nfsv4@linux-nfs.org, linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org To: Neil Brown Return-path: In-Reply-To: <18068.25879.745638.343290@notabene.brown> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org On Wed, 2007-07-11 at 15:05 +1000, Neil Brown wrote: > It just occurred to me: > > If i_version is 64bit, then knfsd would need to be careful when > reading it on a 32bit host. What are the locking rules? How does knfsd use i_version? I would think that if all it was doing was to compare (i_version == previous_version), then locking wouldn't really matter. Well, theoretically, previous_version could be 0x100000000, and i_version could be 0x1ffffffff, knfsd checks the high word, then ext4 updates i_version to 0x200000000, then knfsd checks the low word, detecting no change. How likely is this? (I don't understand why i_version even needs to be 64 bits in the first place.) > Presumably it is only updated under i_mutex protection, but having to > get i_mutex to read it would seem a little heavy handed. How does knfsd protect itself from the inode changing after i_version is checked? Is any locking being done otherwise? > Should it use a seqlock like i_size? > Could we use the same seqlock that i_size uses, or would we need a > separate one? > > NeilBrown -- David Kleikamp IBM Linux Technology Center