Re: lifetime of DCACHE_DISCONECTED dentries

["J. Bruce Fields" <bfields@fieldses.org>]

On Mon, Nov 29, 2010 at 02:56:22PM +1100, Nick Piggin wrote:
> On Tue, Nov 16, 2010 at 5:45 PM, Nick Piggin <npiggin@gmail.com> wrote:
> > On Tue, Nov 16, 2010 at 4:48 AM, J. Bruce Fields <bfields@fieldses.org> wrote:
> >> On Sat, Nov 13, 2010 at 10:53:12PM +1100, Nick Piggin wrote:
> >>> On Sat, Nov 13, 2010 at 5:43 AM, J. Bruce Fields <bfields@fieldses.org> wrote:
> >
> >>> >        - putfh: look up the filehandle.  The only alias found for the
> >>> >          inode will be DCACHE_UNHASHED alias referenced by the filp
> >>> >          associated with the nfsd open.  d_obtain_alias() doesn't like
> >>> >          this, so it creates a new DCACHE_DISCONECTED dentry and
> >>> >          returns that instead.
> >>>
> >>> This seems to be where the thing goes wrong. It isn't a hashed dentry at
> >>> this point here, so d_obtain_alias should not be making one.
> >>
> >> Sounds sensible.  (But can you think of any actual bugs that will result
> >> from trying to add a new hashed dentry in this case?)
> >
> > Well, this one? :)
> >
> >
> >>> I think the inode i_nlink games are much more appropriate on this side of
> >>> the equation, rather than the dput side (after all, d_obtain_alias is setting
> >>> up an alias for the inode).
> >>>
> >>> Can you even put the link check into __d_find_alias?
> >>>
> >>> -               if (S_ISDIR(inode->i_mode) || !d_unhashed(alias)) {
> >>> +               if (S_ISDIR(inode->i_mode) || !inode->i_nlink ||
> >>> !d_unhashed(alias)) {
> >>>
> >>> Something like that?
> >>
> >> The immediate result of that would be for the close rpc (or any rpc's
> >> sent after the file was unlinked) to fail with ESTALE.
> >
> > Why is that? Seems like it would be a bug, because a hashed dentry may
> > be unhashed at any time concurrently to nfsd operation, so it should be
> > able to tolerate that so long as it has a ref on the inode?
> 
> Ping? Did you work out why nfs fails with ESTALE in that case? It seems
> to work in my testing (and do the right thing with freeing the inode).

Bah, sorry, I read too quickly, got the sense of the test backwards, and
thought you were suggesting __d_find_alias() shouldn't return an alias
in the i_nlink == 0 case!

Yes, agreed, that should solve my problem.

But what's the reason for the d_unhashed() check now?  Could we get rid
of it entirely?

--b.
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