Re: [PATCH 07/11] xfs: mark reclaimed inodes invalid earlier

[Dave Chinner <david@fromorbit.com>]

On Thu, Apr 14, 2016 at 08:10:49AM -0400, Brian Foster wrote:
> On Wed, Apr 13, 2016 at 04:49:00PM +1000, Dave Chinner wrote:
> > On Wed, Apr 13, 2016 at 03:31:28PM +1000, Dave Chinner wrote:
> > > From: Dave Chinner <dchinner@redhat.com>
> > > 
> > > The last thing we do before using call_rcu() on an xfs_inode to be
> > > freed is mark it as invalid. This means there is a window between
> > > when we know for certain that the inode is going to be freed and
> > > when we do actually mark it as "freed".
> > > 
> > > This is important in the context of RCU lookups - we can look up the
> > > inode, find that it is valid, and then use it as such not realising
> > > that it is in the final stages of being freed.
> > > 
> > > As such, mark the inode as being invalid the moment we know it is
> > > going to be reclaimed. This can be done while we still hold the
> > > XFS_ILOCK_EXCL and the flush lock in xfs_inode_reclaim, meaning that
> > > it occurs well before we remove it from the radix tree, and that
> > > the i_flags_lock, the XFS_ILOCK and the inode flush lock all act as
> > > synchronisation points for detecting that an inode is about to go
> > > away.
> > > 
> > > For defensive purposes, this allows us to add a further check to
> > > xfs_iflush_cluster to ensure we skip inodes that are being freed
> > > after we grab the XFS_ILOCK_SHARED and the flush lock - we know that
> > > if the inode number if valid while we have these locks held we know
> > > that it has not progressed through reclaim to the point where it is
> > > clean and is about to be freed.
> > > 
> > > [bfoster: fixed __xfs_inode_clear_reclaim() using ip->i_ino after it
> > > 	  had already been zeroed.]
> > 
> > And, of course, in reordering this I dropped this fix because it was
> > handled by the reworking of tagging code to use pag->pag_agno.
> > 
> > So I've brought that small change forward to this patch (using
> > pag->pag_agno instead of deriving it from the ip->i_ino in
> > __xfs_inode_clear_reclaim()).
> > 
> 
> I don't see any such change in this patch..?
> __xfs_inode_clear_reclaim() still uses ip->i_ino.

I meant that I realised that I'd screwed it up and so I'd changed my
local copy after I sent this.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com

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