Re: [PATCH 2/9] vfs: export do_splice_direct() to modules

From: Al Viro
Date: Fri Mar 22 2013 - 14:11:34 EST

On Sat, Mar 23, 2013 at 02:37:55AM +0900, J. R. Okajima wrote:
> David Howells:
> > Now, looking at __sb_start_write(), I'm not entirely sure how the deadlock
> > might operate, so it's possible that this is a false alarm. Maybe Jan Kara can
> > illuminate further, so I've added him to the cc list.
> It is related to the design of UnionMount, isn't it?
> UnionMount is not a filesystem and doen't have its own superblock.
> If it was a fs, then
> - vfs_truncate() acquires sb_writers for the unioning-fs.
> - the unioning-fs may call vfs_truncate() again for the underlying fs.
> - this time, sb_writers is for the underlying fs which is a different
> sb_writers object from the already acquired one.
> So there would be no deadlock.

Doesn't help the situation with copyup - witness overlayfs stepping into the
same deadlock on copyup. It wants ->i_mutex held on directory in upper layer
and it tries to write to file it has created in there. The problem is
with the upper layer superblock getting frozen; having a separate one for
union is irrelevant. Let me check how aufs does... Aha. Your
au_do_copy_file() ends up calling vfs_write() on the file opened in
upper layer. And AFAICS it's called with ->i_mutex held on the directory
in upper layer, so you've got the same deadlock, sorry.
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