Re: [Qemu-devel] [PATCH v2 3/9] jobs: add exit shim

[Max Reitz]

On 2018-08-24 00:08, 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 | 11 +++++++++++
>  job.c              | 18 ++++++++++++++++++
>  2 files changed, 29 insertions(+)

Reviewed-by: Max Reitz <address@hidden>

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