[Qemu-devel] [PATCH v2 00/20] Drain fixes and cleanups, part 3

[Kevin Wolf]

This is the third and hopefully for now last part of my work to fix
drain. The main goal of this series is to make drain robust against
graph changes that happen in any callbacks of in-flight requests while
we drain a block node.

The individual patches describe the details, but the rough plan is to
change all three drain types (single node, subtree and all) to work like
this:

1. First call all the necessary callbacks to quiesce external sources
   for new requests. This includes the block driver callbacks, the child
   node callbacks and disabling external AioContext events. This is done
   recursively.

   Much of the trouble we had with drain resulted from the fact that the
   graph changed while we were traversing the graph recursively. None of
   the callbacks called in this phase may change the graph.

2. Then do a single AIO_WAIT_WHILE() to drain the requests of all
   affected nodes. The aio_poll() called by it is where graph changes
   can happen and we need to be careful.

   However, while evaluating the loop condition, the graph can't change,
   so we can safely call all necessary callbacks, if needed recursively,
   to determine whether there are still pending requests in any affected
   nodes. We just need to make sure that we don't rely on the set of
   nodes being the same between any two evaluation of the condition.

There are a few more smaller, mostly self-contained changes needed
before we're actually safe, but this is the main mechanism that will
help you understand what we're working towards during the series.

v2:

- Rebased on top of current master (e.g. including Job infrastructure)

- Avoid unnecessary parent callbacks for .drained_begin/poll/end:
  * subtree drains: Don't propagate the drain to the parent that we came
                    from recursively
  * drain_all:      Don't propagate the drain to BDS parents (which are
                    already separately drained), but only to non-BDS
                    parents like BBs or Jobs

- Separate bdrv_drain_poll_top_level() function instead of having a
  top_level parameter for bdrv_drain_poll().

- A few commit message and comment improvements


Kevin Wolf (19):
  test-bdrv-drain: bdrv_drain() works with cross-AioContext events
  block: Use bdrv_do_drain_begin/end in bdrv_drain_all()
  block: Remove 'recursive' parameter from bdrv_drain_invoke()
  block: Don't manually poll in bdrv_drain_all()
  tests/test-bdrv-drain: bdrv_drain_all() works in coroutines now
  block: Avoid unnecessary aio_poll() in AIO_WAIT_WHILE()
  block: Really pause block jobs on drain
  block: Remove bdrv_drain_recurse()
  block: Drain recursively with a single BDRV_POLL_WHILE()
  test-bdrv-drain: Test node deletion in subtree recursion
  block: Don't poll in parent drain callbacks
  test-bdrv-drain: Graph change through parent callback
  block: Defer .bdrv_drain_begin callback to polling phase
  test-bdrv-drain: Test that bdrv_drain_invoke() doesn't poll
  block: Allow AIO_WAIT_WHILE with NULL ctx
  block: Move bdrv_drain_all_begin() out of coroutine context
  block: ignore_bds_parents parameter for drain functions
  block: Allow graph changes in bdrv_drain_all_begin/end sections
  test-bdrv-drain: Test graph changes in drain_all section

Max Reitz (1):
  test-bdrv-drain: Add test for node deletion

 include/block/aio-wait.h     |  25 +-
 include/block/block.h        |  31 +-
 include/block/block_int.h    |  14 +
 include/block/blockjob_int.h |   8 +
 block.c                      |  52 +++-
 block/io.c                   | 332 ++++++++++++--------
 block/mirror.c               |   8 +
 block/vvfat.c                |   1 +
 blockjob.c                   |  23 ++
 tests/test-bdrv-drain.c      | 705 +++++++++++++++++++++++++++++++++++++++++--
 10 files changed, 1032 insertions(+), 167 deletions(-)

-- 
2.13.6