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Re: [Qemu-block] [Qemu-devel] [PATCH 3/7] jobs: add exit shim

From: Eric Blake
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH 3/7] jobs: add exit shim
Date: Mon, 20 Aug 2018 16:16:47 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1

On 08/17/2018 02:04 PM, John Snow wrote:
All jobs do the same thing when they leave their running loop:
- Store the return code in a structure
- wait to receive this structure in the main thread
- signal job completion via job_completed

Few jobs do anything beyond exactly this. Consolidate this exit
logic for a net reduction in SLOC.

More seriously, when we utilize job_defer_to_main_loop_bh to call
a function that calls job_completed, job_finalize_single will run
in a context where it has recursively taken the aio_context lock,
which can cause hangs if it puts down a reference that causes a flush.

You can observe this in practice by looking at mirror_exit's careful
placement of job_completed and bdrv_unref calls.

If we centralize job exiting, we can signal job completion from outside
of the aio_context, which should allow for job cleanup code to run with
only one lock, which makes cleanup callbacks less tricky to write.

Signed-off-by: John Snow <address@hidden>
  include/qemu/job.h |  7 +++++++
  job.c              | 19 +++++++++++++++++++
  2 files changed, 26 insertions(+)

Reviewed-by: Eric Blake <address@hidden>

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

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