From mboxrd@z Thu Jan 1 00:00:00 1970 From: Ian Kent Subject: Re: [autofs] [RFC PATCH]autofs4: hang and proposed fix Date: Fri, 18 Nov 2005 09:54:42 -0500 (EST) Message-ID: References: <20051116101740.GA9551@RAM> <1132159817.5720.33.camel@localhost> <1132192362.5720.163.camel@localhost> <437CD7D2.40003@us.ibm.com> <1132259960.5720.177.camel@localhost> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: William H Taber , autofs mailing list , linux-fsdevel Return-path: Received: from wombat.indigo.net.au ([202.0.185.19]:21259 "EHLO wombat.indigo.net.au") by vger.kernel.org with ESMTP id S1751377AbVKRByZ (ORCPT ); Thu, 17 Nov 2005 20:54:25 -0500 To: Ram Pai In-Reply-To: <1132259960.5720.177.camel@localhost> Sender: linux-fsdevel-owner@vger.kernel.org List-Id: linux-fsdevel.vger.kernel.org On Thu, 17 Nov 2005, Ram Pai wrote: > On Thu, 2005-11-17 at 11:19, William H. Taber wrote: > > Ian Kent wrote: > > > On Wed, 16 Nov 2005, Ram Pai wrote: > > > > > >> > > >>The question is: Who is the culprit? stubfs? VFS? or > > >> autofs4? > > > > > > > > > I'm happy to fix it in autofs unless you feel we need to address the wider > > > issue. > > > > > > I'll put together a patch which takes account of this and pushes the > > > hold/release down into try_to_fill_dentry. But I would like a little > > > time to think about whether there may be other implications. > > > > > > > Ian, > > I don't think that you can fix this in the autofs by tinkering with > > holding and releasing the parent i_sem. The reason for this is that you > > don't have any way of knowing if you hold that lock or not. The easy > > case is that nobody holds the lock. But if the lock is held you have no > > way to know that you are the person holding the lock and you cannot > > unlock someone elses lock without serious consequences. > > > > The only way to fix the lock handling is to fix the VFS. This means > > either changing all calls to the d_revalidate functions (or all calls to > > d_revalidate itself) so that the parent i_sem is obtained first, or to > > change lookup_one_len (or actually lookup_hash) to only get the lock > > around the filesystem lookup call, matching what is done in real_lookup. > > I don't know which is better from a locking correctness perspective. > > I would have to defer to the VFS experts on that one. I do know that > > lookup_one_len is called from about 40 places in kernel tree and > > probably from every filesystem outside the tree as well. Either way, it > > is a non-trivial piece of work. > > > > If you take the inconsistant locking as a given, then the fix has to > > involve not doing the d_add on the new dentry until after the mount > > completes. This would eliminate the need for revalidate to wait. You > > would have to provide a mechanism for keeping track of the outstanding > > mount requests and looking for a a mount in progress before starting a > > new request. This would take the waiting out of revalidate and put it > > into the lookup request itself where you are guaranteed that the parent > > i_sem lock is held. > > Even this has a issue I think. Because later when the automounter > attempts to mount, VFS wont' find the corresponding dentry in the dcache > and will allocate a new dentry. And this dentry is not the one which > autofs4 is waiting to be mounted on. No? Yes. The mount triggering depends on the dentry being present. And there is the situation where the mount point directory pre-exists in the autofs (browseable automounts) so lookup is not called. In the new version that I am working on now (when I eventually get it done) directories will pre-exist for all autofs mount points but simply not be displayed based on a mount option. So this could be kinda difficult for me. Ian