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Re: [Qemu-devel] [PATCH 12/14] blockjob: Lie better in child_job_drained

From: Fam Zheng
Subject: Re: [Qemu-devel] [PATCH 12/14] blockjob: Lie better in child_job_drained_poll()
Date: Tue, 11 Sep 2018 16:34:49 +0800
User-agent: Mutt/1.10.1 (2018-07-13)

On Fri, 09/07 18:15, 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.
> Signed-off-by: Kevin Wolf <address@hidden>

Reviewed-by: Fam Zheng <address@hidden>

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