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Re: [Qemu-devel] [PATCH v2 13/17] blockjob: Lie better in child_job_drai

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 13/17] blockjob: Lie better in child_job_drained_poll()
Date: Thu, 13 Sep 2018 23:52:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0

On 13.09.18 14:52, Kevin Wolf wrote:
> Block jobs claim in .drained_poll() that they are in a quiescent state
> as soon as job->deferred_to_main_loop is true. This is obviously wrong,
> they still have a completion BH to run. We only get away with this
> because commit 91af091f923 added an unconditional aio_poll(false) to the
> drain functions, but this is bypassing the regular drain mechanisms.
> However, just removing this and telling that the job is still active
> doesn't work either: The completion callbacks themselves call drain
> functions (directly, or indirectly with bdrv_reopen), so they would
> deadlock then.
> As a better lie, tell that the job is active as long as the BH is
> pending, but falsely call it quiescent from the point in the BH when the
> completion callback is called. At this point, nested drain calls won't
> deadlock because they ignore the job, and outer drains will wait for the
> job to really reach a quiescent state because the callback is already
> running.

...because it's running in the main loop, so it's impossible for another
drain to run there concurrently.  I suppose.

At least that's what makes this patch make sense to me.

> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>  include/qemu/job.h |  3 +++
>  blockjob.c         |  2 +-
>  job.c              | 11 ++++++++++-
>  3 files changed, 14 insertions(+), 2 deletions(-)

Reviewed-by: Max Reitz <address@hidden>

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