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Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: pause block jobs for

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] [Qemu-block] [PATCH v2 0/3] block: pause block jobs for bdrv_drain_begin/end
Date: Fri, 24 Mar 2017 15:27:55 +0000
User-agent: Mutt/1.8.0 (2017-02-23)

On Thu, Mar 23, 2017 at 06:57:14PM +0100, Paolo Bonzini wrote:
> On 23/03/2017 18:44, Stefan Hajnoczi wrote:
> >> It's possible to wedge QEMU if the guest tries to reset a virtio-pci
> >> device as QEMU is also using the drive for a blockjob. This patchset
> >> aims to allow us to safely pause/resume jobs attached to individual
> >> nodes in a manner similar to how bdrv_drain_all_begin/end do.
> > 
> > Weird, I thought the 0 nanosecond sleep that block jobs do in their
> > main loop allows aio_poll() loops to finish.
> The 0 nanosecond sleep is now done in the BDS AioContext rather than in 
> the "non-aio_poll-aware" main loop:
>     commit 0b9caf9b3166c8deb3c4f3a774c2384b069dc29c
>     Author: Fam Zheng <address@hidden>
>     Date:   Tue Aug 26 15:15:43 2014 +0800
>     coroutine: Drop co_sleep_ns
>     block_job_sleep_ns is the only user. Since we are moving towards
>     AioContext aware code, it's better to use the explicit version and drop
>     the old one.
>     Signed-off-by: Fam Zheng <address@hidden>
>     Reviewed-by: BenoƮt Canet <address@hidden>
>     Signed-off-by: Stefan Hajnoczi <address@hidden>

But we hold the AioContext lock and are calling aio_poll(), so I would
expect our loop to terminate.  The blockjob coroutine should still be
leaving this little gap in activity during which the aio_poll() loop


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