xfs_file_splice_read: possible circular locking dependency detected

[Al Viro]

On Thu, Sep 08, 2016 at 09:44:50PM +0100, Al Viro wrote:

> Moreover, I don't see why we need to hold pipe lock the actual call of
> ->read_iter().  Right now we only grab it for "feed into pipe buffers"
> part.  Objections?

Actually, screw the "array of bvec"; we'd need to mark the ones that are
pagecache-backed somehow to tell which methods should be used.  Let's
add a variant of iov_iter that would be backed by pipe_buffer array;
copy_page_to_iter() fills the next slot with an extra reference to the
page we'd been given and using page_cache_pipe_buf_ops for ->ops.
copy_to_iter() adds to the last slot if it has default_pipe_buf_ops for
->ops and still has space in it or allocates a new page, sticks into the
next slot, copies data into it and sets default_pipe_ops for ->ops.

Then all we need is a variant of splice_to_pipe()/splice_{grow,shrink}_spd()
that would work with an array of pipe_buffer instead of page/partial_page
array pair.