[PATCH 1/2] locks: introduce i_blockleases to close lease races

[Mimi Zohar]

On Sun, 2011-06-12 at 15:12 -0400, J. Bruce Fields wrote: 
> On Sun, Jun 12, 2011 at 03:10:04PM -0400, Mimi Zohar wrote:
> > On Sun, 2011-06-12 at 00:08 -0400, J. Bruce Fields wrote:
> > > On Fri, Jun 10, 2011 at 05:34:46PM -0400, J. Bruce Fields wrote:
> > > > On Fri, Jun 10, 2011 at 04:24:00PM -0400, Mimi Zohar wrote:
> > > > > On Thu, 2011-06-09 at 20:10 -0400, J. Bruce Fields wrote:
> > > > > > From: J. Bruce Fields <bfields at redhat.com>
> > > > > > 
> > > > > > Since break_lease is called before i_writecount is incremented, there's
> > > > > > a window between the two where a setlease call would have no way to know
> > > > > > that an open is about to happen.
> > > > > 
> > > > > So unless the break_lease() call is moved from may_open() to after 
> > > > > nameidata_to_filp(), I don't see any other options.
> > > > 
> > > > Actually, offhand I can't see why that wouldn't be OK.
> > > > 
> > > > Though I think we still end up needing something like i_blockleases to
> > > > handle unlink, link, rename, chown, and chmod.
> > > 
> > > Well, I guess there's a bizarre alternative that wouldn't require a new
> > > inode field:
> > 
> > In lieu of adding a new inode field, another possible option, a bit
> > kludgy, would be extending i_flock with an additional fl_flag
> > FL_BLOCKLEASE.
> > 
> > #define IS_BLOCKLEASE(fl)    (fl->fl_flags & FL_BLOCKLEASE)
> 
> Alas, that would mean adding and removing one of these file locks around
> every single link, unlink, rename,....
> 
> --b.

You're adding a call to break_lease() for each of them.  Currently
__break_lease() is only called if a lease exists. Assuming there aren't
any existing leases, couldn't break_lease() call something like
block_lease()?  The free would be after the link, unlink, ...,
completed/failed.

(You wouldn't actually need to alloc/free the 'struct file_lock' each
time, just set the pointer and reset to NULL.)

Mimi

> > 
> > Mimi
> > 
> > > What we care about is conflicts between read leases and operations that
> > > modify the metadata of the inode or the set of names pointing to it.
> > > 
> > > As far as I can tell those operations all take the i_mutex either on the
> > > inode itself or on the parents of one of its aliases.
> > > 
> > > So, you could prevent break_lease/setlease races by calling setlease
> > > under *all* of those i_mutexes:
> > > 
> > > 	- take i_mutex on the inode
> > > 	- take i_lock to prevent the set of aliases from changing
> > > 	- take i_mutex for parent of each alias
> > > 	- set the lease
> > > 	- drop the parent i_mutexes, etc.
> > > 
> > > where the i_mutexes would all be taken with mutex_trylock, and we'd just
> > > fail the whole setlease if any of them failed.
> > > 
> > > ???
> > > 
> > > --b.
> > 
> >