xfs_file_splice_read: possible circular locking dependency detected

[Al Viro]

On Mon, Sep 19, 2016 at 01:08:30PM +1000, Nicholas Piggin wrote:

> Without looking through all the patches again, I believe the issue was
> just that filesystems were not expecting (or at least, not audited to
> expect) pages being added to their pagecache in that particular state
> (they'd expect to go through ->readpage or see !uptodate in prepare_write).
> 
> If some wanted to attach metadata to uptodate pages for example, this
> may have caused a problem. It wasn't some big fundamental problem, just a
> mechanical one.

Umm...  Why not make it non-uptodate/locked, try to replace the original
with it in pagecache and then do full-page ->write_begin immediately
followed by full-page ->write_end?  Looks like that ought to work in
all in-tree cases...