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

[Ian Kent <raven@themaw.net>]

On Wed, 30 Nov 2005, William H. Taber wrote:

> Trond Myklebust wrote:
> > On Wed, 2005-11-30 at 15:32 -0500, William H. Taber wrote:
> > 
> > 
> > > Not only is there this case, but the original premise is wrong as well.
> > > There is a second case in which a d_revalidate function is called with the
> > > parent i_sem and that is when it is called from inside of lookup_one_len.
> > > What makes this tricky is that lookup_one_len is called from
> > > nfs_sillyrename from inside of nfs_rename which is called, naturally
> > > enough by sys_rename.  The rename code is very careful about the order in
> > > which it obtains the parent semaphores because it needs to get two of
> > > them.  It must always obtain the locks in the same order so that does not
> > > get into a deadly embrace.  If we start arbitrarily releasing a parent
> > > semaphore in cached_lookup and taking it again after the revalidate, we
> > > risk breaking the lock ordering and creating a deadly embrace.
> > > 
> > > When I started writing this I thought that it would be safe for the autofs
> > > revalidate code to release the parent semaphore because they do not have a
> > > rename callback.  But I looked again at the rename code and it calls
> > > lookup_hash on the final source and destination files after locking the
> > > parents so the potential for a deadly embrace still exists unless there is
> > > some other assurance that these final lookups will never pend waiting on
> > > the automounter in either their revalidate or lookup routines.  (Actually
> > > the requirement is that they never give up the parent i_sem lock, but the
> > > lookup code has to give up the lock so that the autofs demon can run and
> > > perform the mount so it amounts to the same thing.)
> > > 
> > > The same issue exists for devfs which also releases the parent i_sem lock
> > > so that it can wait inside its revalidation routine.
> > 
> > 
> > So exactly why does autofs4 want to hold the dir->i_sem in d_revalidate
> > in the first place? Can't we move any code that requires dir->i_sem to
> > be held into a ->lookup() method?
> 
> It's not that d_revalidate wants or doesn't want to hold the lock.  The caller
> of lookup_one_len is required to get the lock and this function calls
> lookup_hash which calls cached_lookup which calls d_revalidate.
> 
> > 
> > Trivially, if you have a d_revalidate that does something like
> > 
> > int autofs_revalidate(struct dentry *dentry, struct nameidata *nd)
> > {
> >   d_drop(dentry);
> >   return 0;
> > }
> > 
> > then the VFS will currently allocate a new dentry with the same name,
> > and call ->lookup() on it without dropping dir->i_sem. If you still need
> > to reference the old dentry, then put it on a private list somewhere.
> > That would also allow you to return the old dentry as the result of the
> > ->lookup() operation if that is desirable.
> 
> Problem with that, as I understand it and Ian Kent knows better than I, is
> that the autofs lookup code creates the dentry and fills it in partially and
> marks it as waiting for mounting and wakes up the automount demon.  The demon
> completes the mount and finishes filling in the dentry.  So we cannot have
> some other lookup coming in and removing the dentry on us.  At least that is
> what I understand from Ian's answer when I proposed the same sort of thing to
> him.   Even if  they end up doing something like that in a future version of
> the automounter, I would still like a simple patch that can be applied to
> existing systems as an interim fix.

Lets see if I can keep this explaination simple.

The user space process using the autofs filesystem (autodir or automount) 
needs to be able to call mkdir at mount time as a result of a callback 
from revalidate. Sometimes this comes indirectly from lookup (if the 
directory does not already exist).

lookup_one_len requires the i_sem to be held so two instances of a 
filesystem calling it lead to a deadlock when mkdir is called from 
userspace (the third process). In the case we are discussing this happens 
because the first process calls lookup which releases the i_sem and 
calls revalidate itself. The second calls revalidate which doesn't release 
the i_sem and is places on a wait queue for mount completion. Consequently 
the mkdir blocks.

So the requirement is that autofs release the i_sem during the callback, 
not obtain it.

Will believes that it is not safe for autofs to release i_sem for 
the callback to user space because it is possible that path that aquired 
it may not be the path that has called revalidate and I can see his point.

Never the less I'm still not convinced that this is possible given the 
restrictions of autofs.

Let me try and describe this, hopefully more clearly than I've done so 
far.

The only operations defined for autofs are:

mkdir, rmdir, symlink and unlink 

and the only processes that can do these operations must be in the same 
process group that mounted the filesystem. EACCESS is returned for all 
other processes attempting these operations.

The other functionality is read-only (and perhaps triggers a mount) 
being lookup, revalidate and readdir.

So the question is, can anyone provide an example of a path that, upon 
calling autofs revalidate or lookup with the i_sem held, not be the path 
that aquired it?

Ian