Re: [PATCH for-4.2 01/14] block: remove management of bi_remaining when restoring original bi_end_io

[Mike Snitzer <snitzer@redhat.com>]

On Mon, May 18 2015 at  4:24am -0400,
Christoph Hellwig <hch@infradead.org> wrote:

> In general this looks good.  But as Jan mentioned you need to
> clear BIO_CHAIN when bi_remaining reaches zero

OK I just replied to Jan -- I'm not yet understanding why.  That said, I
also don't have a problem with doing what you guys are asking.  I'd just
like to understand the problem you're forseeing because in practice I'm
not hitting it in testing.

> and I'd really prefer if bio_inc_remaining wuld not be left exported
> and folded into bio_chain so that we prevent new abuses from showing
> up and keep the code centralized.

Your desire to make bio_inc_remaining() private is noted but I think the
proposed blkdev_issue_discard_async() is useful.  In the context of DM
thinp's use of blkdev_issue_discard_async(): yes it is (ab)using
bio_chain() and bio_inc_remaining() to setup the async IO completion
scheme but it makes for a pretty clean solution to the problem of
wanting to have an async interface for discard.