Re: XFS reclaim lock order bug

Dave Chinner
2010-11-25T10:28:17Z

On Thu, Nov 25, 2010 at 07:25:25AM +0100, Peter Zijlstra wrote:
> On Thu, 2010-11-25 at 14:48 +1100, Nick Piggin wrote:
> > On Wed, Nov 24, 2010 at 03:03:41PM -0500, Christoph Hellwig wrote:
> > > On Wed, Nov 24, 2010 at 08:12:58AM +1100, Dave Chinner wrote:
> > > > It is supposed to be handled by the re-initialisation of the
> > > > ip->i_iolock in ->evict_inode (xfs_fs_evict_inode). An inode found
> > > > in the reclaim state must have passed through this reinitialisation,
> > > > so from a lockdep perspective the iolock in the vfs path is a
> > > > different context to the iolock in the reclaim path. That fixed all
> > > > the non-reclaim state related lockdep false positives, so Perhaps
> > > > there is an issue with the lockdep reclaim state checking that does
> > > > not interact well with re-initialised lock contexts?
> > >
> > > I've been looking through this again, and I think it's indeed not
> > > enough. We don't just need to re-initialize it, but also set a
> > > different lockclass for it.
> >
> > Doesn't init_rwsem give it a new class?
>
> Per call-site, yes it should.
>
> > Guys, can you take a quick look at the code Dave is referring to
> > (xfs_fs_evict_inode), and check that it actually does what he
> > intends?
>
> Right, so this is trying to set a different class from the regular init
> site, which (/me applies grep) lives in xfs_inode_alloc(), right?
>
> Ought to work.. assuming the inode will be fully destroyed and new
> inodes are always obtained through xfs_inode_alloc() and not reused.
>
> > We're getting what seems to be false positives in reclaim inversion
> > of lockings. Backtraces here
> > http://oss.sgi.com/pipermail/xfs/2010-November/048092.html
>
> Right, so there its holding the inode in the read path while taking a
> page-fault which does an allocation.
>
> vs
>
> acquiring the inode in the xfs_reclaim_node_shrink() path.
>
>
> Presumably the whole xfs_fs_evict_inode() stuff will happen _after_ its
> possible to end up in that read path?
>
>
> Something like the below would give the lock-class an explicit name,
> because both sites now use the exact same init thing they're called:
>
> (&(&ip->i_iolock)->mr_lock)
> (&(&ip->i_iolock)->mr_lock#2)
>
> Which is hard to tell apart, but I suspect #2 is the dead one, since
> they get numbered in order of appearance and its hard to have a dead
> inode before having a life one ;-)
>
> In that case though, it would suggest the inode got re-used instead of
> destroyed and re-created using xfs_alloc_inode(), is that at all
> possible?

Yes, actually it is - see the XFS_IRECLAIMABLE case in
xfs_iget_cache_hit(). I guess we haven't seen the original lock
inversion false positives that this was supposed to fix because the
reclaim warnings trip first...

I think that means we also need to reinitialise the lock when we recycle
the inode out of the XFS_IRECLAIMABLE state.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com