[PATCH v2 0/3] block: Start/end drain on correct AioContext

[Hanna Reitz <hreitz@redhat.com>]

Hi,

v1 cover letter:
https://lists.nongnu.org/archive/html/qemu-block/2022-09/msg00389.html

bdrv_replace_child_noperm() drains the child via
bdrv_parent_drained_{begin,end}_single().  When it removes a child, the
bdrv_parent_drained_end_single() at its end will be called on an empty
child, making the BDRV_POLL_WHILE() in it poll the main AioContext
(because c->bs is NULL).

That’s wrong, though, because it’s supposed to operate on the parent.
bdrv_parent_drained_end_single_no_poll() will have scheduled any BHs in
the parents’ AioContext, which may be anything, not necessarily the main
context.  Therefore, we must poll the parent’s context.

Patch 3 does this for both bdrv_parent_drained_{begin,end}_single().
Patch 1 ensures that we can legally call
bdrv_child_get_parent_aio_context() from those I/O context functions,
and patch 2 fixes blk_do_set_aio_context() to not cause an assertion
failure if it beginning a drain can end up in blk_get_aio_context()
before blk->ctx has been updated.


v2:
- Patch 1:
  - Move bdrv_child_get_parent_aio_context() from block-global-state.h
    to block-io.h
  - Move BdrvChildClass.get_parent_aio_context into the I/O code section
  - Mark blk_root_get_parent_aio_context() as I/O code
  - Mark child_job_get_parent_aio_context(), and lock the job mutex,
    which we now need to do (as of 3ed4f708fe1)
- Patch 2:
  - Added comment similar to Kevin’s suggestion
    (Still decided to take Kevin’s R-b, even though I didn’t use the
    literal suggestion, but made it a bit more verbose)


Hanna Reitz (3):
  block: Make bdrv_child_get_parent_aio_context I/O
  block-backend: Update ctx immediately after root
  block: Start/end drain on correct AioContext

 include/block/block-global-state.h | 1 -
 include/block/block-io.h           | 2 ++
 include/block/block_int-common.h   | 4 ++--
 block.c                            | 2 +-
 block/block-backend.c              | 9 ++++++++-
 block/io.c                         | 6 ++++--
 blockjob.c                         | 3 ++-
 7 files changed, 19 insertions(+), 8 deletions(-)

-- 
2.36.1