Re: [PATCH] fs/dcache: allow __d_obtain_alias() to return unhashed dentries

[NeilBrown <neilb@xxxxxxx>]

On Fri, 29 Jun 2012 16:29:03 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx>
wrote:

> On Fri, Jun 29, 2012 at 04:10:34PM -0400, J. Bruce Fields wrote:
> > On Thu, Jun 28, 2012 at 09:59:27AM -0400, J. Bruce Fields wrote:
> > > Coming back to this now, just trying to review the
> > > filehandle-lookup/dcache interactions:
> > > 
> > > On Fri, Mar 11, 2011 at 03:07:49PM +1100, NeilBrown wrote:
> > > > 1/ Originally DCACHE_DISCONNECTED didn't really mean much - it's presence
> > > >    was only a hint, its absence was a strong statement.
> > > >    If the flag is set, the dentry might not be linked to the root.
> > > >    If it is clear, it definitely is link through to the root.
> > > >    However I think it was used with stronger intent than that.
> > > > 
> > > >    Now it seems to mean a little bit more:  If it is set and the dentry
> > > >    is hashed, then it must be on the sb->s_anon list.
> > > 
> > > The code that makes that assumption is __d_shrink (which does the work
> > > of d_drop)--it uses DCACHE_DISCONECTED to decide which hash chain to
> > > lock.
> > > 
> > > I can't find any basis for that assumption.  The only code that clears
> > > DCACHE_DISCONNECTED is in expfs.c, and it isn't done at the same time as
> > > hashing.  Am I missing something?
> > > 
> > > >    This is a significant
> > > >    which I never noticed (I haven't been watching).  Originally a
> > > >    disconnected dentry would be attached (and hashed) to its parent.  Then
> > > >    that parent would get its own parent and so on until it was attached all
> > > >    the way to the root.  Only then would be start clearing
> > > >    DCACHE_DISCONNECTED.  It seems we must clear it sooner now... I wonder if
> > > >    that is correct.
> > > 
> > > It looks wrong to me:
> > > 
> > > If we clear DCACHE_DISCONNECTED too early, then we risk a filehandle
> > > lookup thinking the dentry is OK to use.  That could mean for example
> > > trying to rename across directories that don't have any ancestor
> > > relationship to each other in the dcache yet.
> > > 
> > > So we need to wait to clear DCACHE_DISCONNECTED until we *know* the
> > > dentry's parents go all the way back to the root.  As you say, that's
> > > what the current code does.
> > > 
> > > But that means DCACHE_DISCONNECTED dentries can be hashed to their
> > > parents, and __d_shrink can be handed such dentries and then get the
> > > locking wrong.
> > > 
> > > It looks like this bug might originate with Nick Piggin's ceb5bdc2d246
> > > "fs: dcache per-bucket dcache hash locking"?  There's no discussion in
> > > the changelog, so probably it was just based on an unexamined assumption
> > > about DCACHE_DISCONNECTED.
> > > 
> > > I wonder if an IS_ROOT() test could replace the DCACHE_DISCONNECTED test
> > > in __d_shrink(), or if we need another flag, or ?
> > 
> > Bah, sorry, and I only just noticed that you already said as much later
> > and did the IS_ROOT() thing in your patch.
> > 
> > Anyway, here's just that one change with a slightly more painstaking
> > changelog.
> > 
> > --b.
> > 
> > commit b1fa644355122627424fe2240a9fc60cbef4c349
> > Author: J. Bruce Fields <bfields@xxxxxxxxxx>
> > Date:   Thu Jun 28 12:10:55 2012 -0400
> > 
> >     dcache: use IS_ROOT to decide where dentry is hashed
> >     
> >     Every hashed dentry is either hashed in the dentry_hashtable, or a
> >     superblock's s_anon list.
> >     
> >     __d_shrink assumes it can determine which is the case by checking
> >     DCACHE_DISCONNECTED; this is not true.
> >     
> >     It is true that when DCACHE_DISCONNECTED is cleared, the dentry is not
> >     only hashed on dentry_hashtable, but is fully connected to its parents
> >     back to the root.
> >     
> >     But the converse is *not* true: fs/exportfs/expfs.c:reconnect_path()
> >     attempts to connect a directory (found by filehandle lookup) back to
> >     root by ascending to parents and performing lookups one at a time.  It
> >     does not clear DCACHE_DISCONNECTED until its done, and that is not at
> >     all an atomic process.
> >     
> >     In particular, it is possible for DCACHE_DISCONNECTED to be set on a
> >     dentry which is hashed on the dentry_hashtable.
> >     
> >     Instead, use IS_ROOT() to check which hash chain a dentry is on.  This
> >     *does* work:
> >     
> >     Dentries are hashed only by:
> >     
> >     	- d_obtain_alias, which adds an IS_ROOT() dentry to sb_anon.
> >     
> >     	- __d_rehash, called by _d_rehash: hashes to the dentry's
> >     	  parent, and all callers of _d_rehash appear to have d_parent
> >     	  set to a "real" parent.
> >     	- __d_rehash, called by __d_move: rehashes the moved dentry to
> >     	  hash chain determined by target, and assigns target's d_parent
> >     	  to its d_parent, before dropping the dentry's d_lock.
> >     
> >     Therefore I believe it's safe for a holder of a dentry's d_lock to
> >     assume that it is hashed on sb_anon if and only if IS_ROOT(dentry) is
> >     true.
> >     
> >     I believe the incorrect assumption about DCACHE_DISCONNECTED was
> >     originally introduced by ceb5bdc2d246 "fs: dcache per-bucket dcache hash
> >     locking".
> >     
> >     Cc: Neil Brown <neilb@xxxxxxx>
> >     Cc: Nick Piggin <npiggin@xxxxxxxxx>
> >     Signed-off-by: J. Bruce Fields <bfields@xxxxxxxxxx>
> > 
> > diff --git a/fs/dcache.c b/fs/dcache.c
> > index 87c2da7..b2b382c 100644
> > --- a/fs/dcache.c
> > +++ b/fs/dcache.c
> > @@ -410,7 +410,7 @@ static void __d_shrink(struct dentry *dentry)
> >  {
> >  	if (!d_unhashed(dentry)) {
> >  		struct hlist_bl_head *b;
> > -		if (unlikely(dentry->d_flags & DCACHE_DISCONNECTED))
> > +		if (unlikely(IS_ROOT(dentry->d_flags)))
> 
> Um, right--I'll send an actual tested version along with some other
> stuff later.

:-)

If that tested version looks like:
                if (unlikely(IS_ROOT(dentry)))
you can add a 
   Reviewed-by: NeilBrown <neilb@xxxxxxx>

Thanks,
NeilBrown


> 
> --b.
> 
> >  			b = &dentry->d_sb->s_anon;
> >  		else
> >  			b = d_hash(dentry->d_parent, dentry->d_name.hash);
> --
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