Re: [GIT PULL] 9p file system bug fixes for 2.6.35-rc2

Al Viro
2010-06-08T00:41:14Z

On Mon, Jun 07, 2010 at 05:08:19PM -0700, Linus Torvalds wrote:

> In fact, the other thing that I find doing that whole "dentry->d_parent"
> thing seems to literally be broken. If you look at v9fs_fid_lookup(),
> you'll notice how it walks up the d_parent chain, and at that point you do
> NOT own the directory i_mutex, so at that point d_parent really _can_ be
> changing wildly due to concurrent renames or whatever.

Eh... It's bogus, all right, but i_mutex is not the correct solution.
You'd have to take it on a lot of inodes along the way to root *and*
you'd violate the ordering in process (ancestors first).

I'm not sure what's the right thing to do there, actually - s_vfs_rename_sem
also won't do, since it'll give you ordering problems of its own (it's
taken before i_mutex in VFS, so trying to take it under i_mutex would not
do).

The _really_ interesting question is how do servers deal with topology-changing
renames. Note that the problem exists only with extended 9P - with the
original one all of that had been a non-issue, since it didn't allow
cross-directory renames at all and the tree topology remained stable all along.