xfs_file_splice_read: possible circular locking dependency detected

[Al Viro]

On Thu, Sep 08, 2016 at 11:29:11AM -0400, CAI Qian wrote:

> > Nope, this goes back to the splice rework back around ~3.16, IIRC.
> Right. FYI, revert the commit below fixes the regression,
> 
> 8d02076 : ->splice_write() via ->write_iter()

... and brings back a lot of other crap.  The thing is, pipe lock should
be on the outside of everything fs might be taking, so that splice IO
is the same as normal IO as far as filesystem locking is concerned.  For
the write side it had been done in that commit, for the read side it's yet
to be done.