Re: NFS lockdep lock misordering mmap_sem<->i_mutex_key with 2.6.32-git1

[Andi Kleen <andi@firstfloor.org>]

On Wed, Dec 16, 2009 at 08:09:51AM -0500, Trond Myklebust wrote:
> On Wed, 2009-12-16 at 01:53 +0100, Andi Kleen wrote: 
> > > If you want to work around the problem rather than going for something
> > 
> > I am mostly interested in making the ugly warning on my systems
> > go away, preferably without breaking anything in the process.
> > Whatever works.
> > 
> > > like Peter's split up of the mmap() callback, then I'd suggest changing
> > > to using nfs_revalidate_mapping_nolock() instead. The fact that we are
> > > seeing these lock misordering warnings is proof that the call to
> > > nfs_revalidate_mapping() is not always a no-op.
> > 
> > I would say the interesting question is if there is really a expectation
> > that mmap does this kind of synchronization? 
> 
> Usually people who set the 'noac' mount flag will expect these syscalls
> to act as synchronisation points.

It would be definitely a synchronization point if they deadlock in 
mmap due to a ABBA race.

> Typically, their applications will be using some kind of locking scheme
> that does not require POSIX or BSD locks to be set. For instance, they
> may synchronise by means of a token passed through a socket (common in
> MPI iirc).

Still mmap seems like an odd synchronization point. Is not doing it 
in it really likely to break anything?

> > Why in mmap, not somewhere else?
> 
> We do the same thing in the read() and write() syscalls.

Ok I didn't fully understand your suggestion to use the _nolock
variant. Are you saying i_mutex is sometimes not needed?

I thought _nolock was only for the case i_mutex is already hold --
which is not the case here.

-Andi
-- 
ak@linux.intel.com -- Speaking for myself only.