Return-Path: Received: from mail-ww0-f44.google.com ([74.125.82.44]:50449 "EHLO mail-ww0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754391Ab0K3BAT convert rfc822-to-8bit (ORCPT ); Mon, 29 Nov 2010 20:00:19 -0500 In-Reply-To: <20101129193248.GA9897@fieldses.org> References: <20101112184353.GA32745@fieldses.org> <20101115174837.GB10044@fieldses.org> <20101129193248.GA9897@fieldses.org> Date: Tue, 30 Nov 2010 12:00:16 +1100 Message-ID: Subject: Re: lifetime of DCACHE_DISCONECTED dentries From: Nick Piggin To: "J. Bruce Fields" Cc: linux-nfs@vger.kernel.org, linux-fsdevel@vger.kernel.org Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-nfs-owner@vger.kernel.org List-ID: MIME-Version: 1.0 On Tue, Nov 30, 2010 at 6:32 AM, J. Bruce Fields wrote: > On Mon, Nov 29, 2010 at 02:56:22PM +1100, Nick Piggin wrote: >> On Tue, Nov 16, 2010 at 5:45 PM, Nick Piggin wrote: >> > On Tue, Nov 16, 2010 at 4:48 AM, J. Bruce Fields wrote: >> >> On Sat, Nov 13, 2010 at 10:53:12PM +1100, Nick Piggin wrote: >> >>> On Sat, Nov 13, 2010 at 5:43 AM, J. Bruce Fields wrote: >> > >> >>> > ? ? ? ?- putfh: look up the filehandle. ?The only alias found for the >> >>> > ? ? ? ? ?inode will be DCACHE_UNHASHED alias referenced by the filp >> >>> > ? ? ? ? ?associated with the nfsd open. ?d_obtain_alias() doesn't like >> >>> > ? ? ? ? ?this, so it creates a new DCACHE_DISCONECTED dentry and >> >>> > ? ? ? ? ?returns that instead. >> >>> >> >>> This seems to be where the thing goes wrong. It isn't a hashed dentry at >> >>> this point here, so d_obtain_alias should not be making one. >> >> >> >> Sounds sensible. ?(But can you think of any actual bugs that will result >> >> from trying to add a new hashed dentry in this case?) >> > >> > Well, this one? :) >> > >> > >> >>> I think the inode i_nlink games are much more appropriate on this side of >> >>> the equation, rather than the dput side (after all, d_obtain_alias is setting >> >>> up an alias for the inode). >> >>> >> >>> Can you even put the link check into __d_find_alias? >> >>> >> >>> - ? ? ? ? ? ? ? if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) { >> >>> + ? ? ? ? ? ? ? if (S_ISDIR(inode->i_mode) || !inode->i_nlink || >> >>> !d_unhashed(alias)) { >> >>> >> >>> Something like that? >> >> >> >> The immediate result of that would be for the close rpc (or any rpc's >> >> sent after the file was unlinked) to fail with ESTALE. >> > >> > Why is that? Seems like it would be a bug, because a hashed dentry may >> > be unhashed at any time concurrently to nfsd operation, so it should be >> > able to tolerate that so long as it has a ref on the inode? >> >> Ping? Did you work out why nfs fails with ESTALE in that case? It seems >> to work in my testing (and do the right thing with freeing the inode). > > Bah, sorry, I read too quickly, got the sense of the test backwards, and > thought you were suggesting __d_find_alias() shouldn't return an alias > in the i_nlink == 0 case! > > Yes, agreed, that should solve my problem. OK, good. > But what's the reason for the d_unhashed() check now? ?Could we get rid > of it entirely? Well when the inode still has links I think we actually do want any new references to go to hashed dentries. Definitely for d_splice_alias.