Re: [Qemu-block] [PATCH 0/5] blockjobs: Fix transactional race condition

[John Snow]

I should also clarify that this is ambiguously for either 2.7 or 2.8.

2.7: This fixes a real, observable problem with transactional completion 
that has the capacity to hang QEMU or segfault due to QLIST corruption.

2.8: Incremental backup is not earnestly a supported feature yet as 
persistence and migration are not yet integrated, so perhaps it's not a 
game-breaker that this feature breaks in some circumstances.


On 08/08/2016 03:09 PM, John Snow wrote:
> There are a few problems with transactional job completion right now.
>
> First, if jobs complete so quickly they complete before remaining jobs
> get a chance to join the transaction, the completion mode can leave well
> known state and the QLIST can get corrupted and the transactional jobs
> can complete in batches or phases instead of all together.
>
> Second, if two or more jobs defer to the main loop at roughly the same
> time, it's possible for one job's cleanup to directly invoke the other
> job's cleanup from within the same thread, leading to a situation that
> will deadlock the entire transaction.
>
> Thanks to Vladimir for pointing out these modes of failure.
>
> ________________________________________________________________________________
>
> For convenience, this branch is available at:
> https://github.com/jnsnow/qemu.git branch job-manual-start
> https://github.com/jnsnow/qemu/tree/job-manual-start
>
> This version is tagged job-manual-start-v1:
> https://github.com/jnsnow/qemu/releases/tag/job-manual-start-v1
>
> John Snow (4):
>   blockjob: add block_job_start
>   blockjob: refactor backup_start as backup_job_create
>   blockjob: add .clean property
>   iotests: add transactional failure race test
>
> Vladimir Sementsov-Ogievskiy (1):
>   blockjob: fix dead pointer in txn list
>
>  block/backup.c             |  50 +++++++-----
>  block/commit.c             |   2 +-
>  block/mirror.c             |   2 +-
>  block/stream.c             |   2 +-
>  blockdev.c                 | 194 ++++++++++++++++++++++++++-------------------
>  blockjob.c                 |  24 +++++-
>  include/block/block_int.h  |  19 ++---
>  include/block/blockjob.h   |  16 ++++
>  tests/qemu-iotests/124     |  91 +++++++++++++++++++++
>  tests/qemu-iotests/124.out |   4 +-
>  tests/test-blockjob-txn.c  |   2 +-
>  11 files changed, 284 insertions(+), 122 deletions(-)

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—js