[Qemu-devel] [PATCH 0/4] block: Keep track of parent quiescing

[Max Reitz]

We currently do not keep track of how many times a child has quiesced
its parent.  We just guess based on the child’s quiesce_counter.  That
keeps biting us when we try to leave drained sections or detach children
(see e.g. commit 5cb2737e925042e).

I think we need an explicit counter to keep track of how many times a
parent has been quiesced (patch 1).  That just makes the code more
resilient.

Actually, no, we don’t need a counter, we need a boolean.  See patch 2
for the explanation.

Yes, it is a bit weird to introduce a counter first (patch 1) and then
immediately make it a boolean (patch 2).  But I believe this to be the
most logical change set.

(“Our current model relies on counting, so adding an explicit counter
makes sense.  It then turns out that counting is probably not the best
idea, so why not make it a boolean?”)


Max Reitz (4):
  block: Introduce BdrvChild.parent_quiesce_counter
  block: Make @parent_quiesced a bool
  iotests: Add @has_quit to vm.shutdown()
  iotests: Test commit with a filter on the chain

 include/block/block.h      |  9 +++---
 include/block/block_int.h  |  6 ++++
 block.c                    | 18 ++++-------
 block/io.c                 | 61 +++++++++++++++++++++++++++++---------
 tests/test-bdrv-drain.c    | 31 +++++++++++--------
 python/qemu/__init__.py    |  5 ++--
 tests/qemu-iotests/040     | 40 ++++++++++++++++++++++++-
 tests/qemu-iotests/040.out |  4 +--
 tests/qemu-iotests/255     |  2 +-
 9 files changed, 126 insertions(+), 50 deletions(-)

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2.21.0