Re: [PATCH 3/3] nfs: use ->mmap_prepare() to avoid an AB-BA deadlock

[Peter Zijlstra <a.p.zijlstra@chello.nl>]

On Wed, 2007-11-14 at 16:41 -0500, Trond Myklebust wrote:
> On Wed, 2007-11-14 at 22:31 +0100, Peter Zijlstra wrote:
> > On Wed, 2007-11-14 at 22:22 +0100, Nick Piggin wrote:
> > > On Wed, Nov 14, 2007 at 09:01:39PM +0100, Peter Zijlstra wrote:
> > > > Normal locking order is:
> > > > 
> > > >   i_mutex
> > > >     mmap_sem
> > > > 
> > > > However NFS's ->mmap hook, which is called under mmap_sem, can take i_mutex.
> > > > Avoid this potential deadlock by doing the work that requires i_mutex from
> > > > the new ->mmap_prepare().
> > > > 
> > > > [ Is this sufficient, or does it introduce a race? ]
> > > 
> > > Seems like an OK patchset in my opinion. I don't know much about NFS
> > > unfortunately, but I wonder what prevents the condition fixed by
> > > nfs_revalidate_mapping from happening again while the mmap is active...?
> > 
> > As the changelog might have suggested, I'm not overly sure of the nfs
> > requirements myself. I think it just does a best effort at getting the
> > pages coherent with other clients, and then hopes for the best.
> > 
> > I'll let Trond enlighten us further before I make an utter fool of
> > myself :-)
> 
> The NFS client needs to check the validity of already cached data before
> it allows those pages to be mmapped. If it finds out that the cache is
> stale, then we need to call invalidate_inode_pages2() to clear out the
> cache and refresh it from the server. The inode->i_mutex is necessary in
> order to prevent races between the new writes and the cache invalidation
> code.

Right, but I guess what Nick asked is, if pages could be stale to start
with, how is that avoided in the future.

The way I understand it, this re-validate is just a best effort at
getting a coherent image.