[v2,00/16] block: remove aio_disable_external() API

Message

Stefan Hajnoczi April 19, 2023, 5:28 p.m. UTC

v2:
- Do not rely on BlockBackend request queuing, implement .drained_begin/end()
  instead in xen-block, virtio-blk, and virtio-scsi [Paolo]
- Add qdev_is_realized() API [Philippe]
- Add patch to avoid AioContext lock around blk_exp_ref/unref() [Paolo]
- Add patch to call .drained_begin/end() from main loop thread to simplify
  callback implementations

The aio_disable_external() API temporarily suspends file descriptor monitoring
in the event loop. The block layer uses this to prevent new I/O requests being
submitted from the guest and elsewhere between bdrv_drained_begin() and
bdrv_drained_end().

While the block layer still needs to prevent new I/O requests in drained
sections, the aio_disable_external() API can be replaced with
.drained_begin/end/poll() callbacks that have been added to BdrvChildClass and
BlockDevOps.

This newer .bdrained_begin/end/poll() approach is attractive because it works
without specifying a specific AioContext. The block layer is moving towards
multi-queue and that means multiple AioContexts may be processing I/O
simultaneously.

The aio_disable_external() was always somewhat hacky. It suspends all file
descriptors that were registered with is_external=true, even if they have
nothing to do with the BlockDriverState graph nodes that are being drained.
It's better to solve a block layer problem in the block layer than to have an
odd event loop API solution.

The approach in this patch series is to implement BlockDevOps
.drained_begin/end() callbacks that temporarily stop file descriptor handlers.
This ensures that new I/O requests are not submitted in drained sections.

The first two virtio-scsi patches were already sent as a separate series. I
included them because they are necessary in order to fully remove
aio_disable_external().

Based-on: 087bc644b7634436ca9d52fe58ba9234e2bef026 (kevin/block-next)

Stefan Hajnoczi (16):
  hw/qdev: introduce qdev_is_realized() helper
  virtio-scsi: avoid race between unplug and transport event
  virtio-scsi: stop using aio_disable_external() during unplug
  block/export: only acquire AioContext once for
    vhost_user_server_stop()
  util/vhost-user-server: rename refcount to in_flight counter
  block/export: wait for vhost-user-blk requests when draining
  block/export: stop using is_external in vhost-user-blk server
  hw/xen: do not use aio_set_fd_handler(is_external=true) in
    xen_xenstore
  block: add blk_in_drain() API
  block: drain from main loop thread in bdrv_co_yield_to_drain()
  xen-block: implement BlockDevOps->drained_begin()
  hw/xen: do not set is_external=true on evtchn fds
  block/export: rewrite vduse-blk drain code
  block/export: don't require AioContext lock around blk_exp_ref/unref()
  block/fuse: do not set is_external=true on FUSE fd
  virtio: make it possible to detach host notifier from any thread

 hw/block/dataplane/xen-block.h              |   2 +
 include/block/export.h                      |   2 +
 include/hw/qdev-core.h                      |  17 ++-
 include/qemu/vhost-user-server.h            |   8 +-
 include/sysemu/block-backend-common.h       |  25 ++--
 include/sysemu/block-backend-global-state.h |   1 +
 block/block-backend.c                       |   7 ++
 block/export/export.c                       |  13 +-
 block/export/fuse.c                         |  58 ++++++++-
 block/export/vduse-blk.c                    | 128 ++++++++++++++------
 block/export/vhost-user-blk-server.c        |  73 +++++++----
 block/io.c                                  |   3 +-
 hw/block/dataplane/virtio-blk.c             |   2 +
 hw/block/dataplane/xen-block.c              |  42 +++++--
 hw/block/xen-block.c                        |  24 +++-
 hw/i386/kvm/xen_xenstore.c                  |   2 +-
 hw/scsi/scsi-bus.c                          |   6 +-
 hw/scsi/scsi-disk.c                         |   1 +
 hw/scsi/virtio-scsi-dataplane.c             |   9 ++
 hw/scsi/virtio-scsi.c                       |  21 ++--
 hw/xen/xen-bus.c                            |  11 +-
 util/vhost-user-server.c                    |  37 +++---
 22 files changed, 348 insertions(+), 144 deletions(-)