Re: [Qemu-devel] [PATCH v3] throttle-groups: cancel timers on restart

[Eric Blake <eblake@redhat.com>]

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On 09/25/2017 08:57 AM, Stefan Hajnoczi wrote:
> Throttling group members are restarted on config change and by
> bdrv_drained_begin().  Pending timers should be cancelled before
> restarting the queue, otherwise requests see that tg->any_timer_armed[]
> is already set and do not schedule a timer.
> 
> For example, a hang occurs at least since QEMU 2.10.0 with -drive
> iops=100 because no further timers will be scheduled:
> 
>   (guest)$ dd if=/dev/zero of=/dev/vdb oflag=direct count=1000
>   (qemu) stop
>   (qemu) cont
>   ...I/O is stuck...
> 
> This can be fixed by calling throttle_group_detach_aio_context() from a
> bdrv_drained_begin/end() region.  This way timers are quiesced properly,
> requests are drained, and other throttle group members are scheduled, if
> necessary.
> 
> Reported-by: Yongxue Hong <yhong@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---

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

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


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