Re: [2.4] VFS locking problem during concurrent link/unlink

[Chris Mason]

On Thu, 2003-01-16 at 10:43, Oleg Drokin wrote:
> Hello!
> 
> On Thu, Jan 16, 2003 at 10:39:41AM -0500, Chris Mason wrote:
> > >    Debugging reiserfs problem that can be demonstrated with script created by
> > >    Zygo Blaxell, I started to wonder if the problem presented is indeed reiserfs
> > >    fault and not VFS.
> > >    Though the Zygo claims script only produces problems on reiserfs, I am trying
> > >    it now myself on ext2 (which will take some time).
> > > 
> > >    Debugging shows that reiserfs_link is sometimes called for inodes whose
> > >    i_nlink is zero (and all corresponding data is deleted already).
> > >    So my current guess of what's going on is this:
> > No, this is a reiserfs bug, since we schedule after doing link checks in
> > reiserfs_link and reiserfs_unlink.  I sent a patch to reiserfs dev a
> > while ago, I'll pull it out of the suse kernel and rediff against
> > 2.4.20.
> 
> Yes we do.
> But on the other hand I've put a check at the beginning of reiserfs_link
> and I am still seeing these links on inodes with i_nlink == 0.

That's because we don't inc the link count in reiserfs_link before we
schedule.  The bug works a little like this:

link count at 1
reiserfs_link: make new directory entry for link, schedule()
reiserfs_unlink: dec link count to zero, remove file stat data
reiserfs_link: inc link count, return thinking the stat data is still
there

All of which leads to expanding chaos as we process this link pointing
to nowhere but still have a valid in ram inode pointing to it.

-chris