Re: [autofs] [RFC PATCH]autofs4: hang and proposed fix

[Ian Kent <raven@themaw.net>]

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