[Qemu-devel] [RFC 00/13] aio: drop io_flush()

[Stefan Hajnoczi <stefanha@redhat.com>]

Here's my entry to the "let's get rid of io_flush()" effort.  It's based on
Paolo's insight about bdrv_drain_all() that the block layer already has a
tracked_requests list.  io_flush() is redundant since the block layer already
knows if requests are pending.

The point of this effort is to simplify our event loop(s).  If we can reduce
custom features like io_flush() it becomes easier to unify event loops and
reuse glib or other options.

This is also important to me for dataplane, since bdrv_drain_all() is one of
the synchronization points between threads.  QEMU monitor commands invoke
bdrv_drain_all() while the block device is accessed from a dataplane thread.

Background on io_flush() semantics:

The io_flush() handler must return 1 if this aio fd handler is active.  That
is, requests are pending and we'd like to make progress by monitoring the fd.

If io_flush() returns 0, the aio event loop skips monitoring this fd.  This is
critical for block drivers like iscsi where we have an idle TCP socket which we
want to block on *only* when there are pending requests.

The series works as follows:

1. Make bdrv_drain_all() use tracked_requests to determine when to stop
   waiting.  From this point onwards io_flush() is redundant.

2. Drop io_flush() handlers and block driver internal state (e.g. in_flight
   counters).  Just pass NULL for the io_flush argument.

3. Drop io_flush argument from aio_set_fd_handler() and related functions.

I split Step 2 from Step 3 so that experts in sheepdog, rbd, curl, etc can
review those patches in isolation.  Otherwise we'd have a monster patch that
touches all files and is hard to review.

Stefan Hajnoczi (13):
  block: stop relying on io_flush() in bdrv_drain_all()
  dataplane/virtio-blk: check exit conditions before aio_poll()
  aio: stop using .io_flush()
  block/curl: drop curl_aio_flush()
  block/gluster: drop qemu_gluster_aio_flush_cb()
  block/iscsi: drop iscsi_process_flush()
  block/linux-aio: drop qemu_laio_completion_cb()
  block/nbd: drop nbd_have_request()
  block/rbd: drop qemu_rbd_aio_flush_cb()
  block/sheepdog: drop have_co_req() and aio_flush_request()
  dataplane/virtio-blk: drop flush_true() and flush_io()
  thread-pool: drop thread_pool_active()
  aio: drop io_flush argument

 aio-posix.c                     | 31 ++++---------------------------
 aio-win32.c                     | 26 +++-----------------------
 async.c                         |  4 ++--
 block.c                         | 28 ++++++++++++++++++----------
 block/curl.c                    | 25 ++++---------------------
 block/gluster.c                 | 21 +++------------------
 block/iscsi.c                   | 10 +---------
 block/linux-aio.c               | 18 ++----------------
 block/nbd.c                     | 18 ++++--------------
 block/rbd.c                     | 16 ++--------------
 block/sheepdog.c                | 33 ++++++++-------------------------
 hw/block/dataplane/virtio-blk.c | 25 ++++++-------------------
 include/block/aio.h             |  8 ++------
 main-loop.c                     |  9 +++------
 thread-pool.c                   | 11 ++---------
 15 files changed, 64 insertions(+), 219 deletions(-)

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1.8.1.4