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

[Eric Blake <eblake@redhat.com>]

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 <jsnow@redhat.com>
> ---
>   include/qemu/job.h |  7 +++++++
>   job.c              | 19 +++++++++++++++++++
>   2 files changed, 26 insertions(+)
> 

Reviewed-by: Eric Blake <eblake@redhat.com>

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