On Thu, Jan 30, 2020 at 07:23:42PM -0800, Bijan Mottahedeh wrote:

Get a reference to a bio, so it won't be freed if end_io() gets to
it before submit_io() returns.  Defer the release of the first bio
in a mult-bio request until the last end_io() since the first bio is
embedded in the dio structure and must therefore persist through an
entire multi-bio request.

Can you explain the issue a little more?

The initial bio is embedded into the dio, and will have a reference
until the bio_put call at the end of the function, so we can't have
a race for that one and won't ever need the refcount for the single
bio case.  Avoiding the atomic is pretty important for aio/uring
performance.