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

[Mimi Zohar]

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)

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.