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Re: [Qemu-block] [PATCH v3] aio: add missing aio_notify() to aio_enable_


From: Stefan Hajnoczi
Subject: Re: [Qemu-block] [PATCH v3] aio: add missing aio_notify() to aio_enable_external()
Date: Tue, 9 May 2017 11:06:45 -0400
User-agent: Mutt/1.8.0 (2017-02-23)

On Mon, May 08, 2017 at 02:07:05PM -0400, Stefan Hajnoczi wrote:
> The main loop uses aio_disable_external()/aio_enable_external() to
> temporarily disable processing of external AioContext clients like
> device emulation.
> 
> This allows monitor commands to quiesce I/O and prevent the guest from
> submitting new requests while a monitor command is in progress.
> 
> The aio_enable_external() API is currently broken when an IOThread is in
> aio_poll() waiting for fd activity when the main loop re-enables
> external clients.  Incrementing ctx->external_disable_cnt does not wake
> the IOThread from ppoll(2) so fd processing remains suspended and leads
> to unresponsive emulated devices.
> 
> This patch adds an aio_notify() call to aio_enable_external() so the
> IOThread is kicked out of ppoll(2) and will re-arm the file descriptors.
> 
> The bug can be reproduced as follows:
> 
>   $ qemu -M accel=kvm -m 1024 \
>          -object iothread,id=iothread0 \
>          -device virtio-scsi-pci,iothread=iothread0,id=virtio-scsi-pci0 \
>          -drive 
> if=none,id=drive0,aio=native,cache=none,format=raw,file=test.img \
>          -device scsi-hd,id=scsi-hd0,drive=drive0 \
>          -qmp tcp::5555,server,nowait
> 
>   $ scripts/qmp/qmp-shell localhost:5555
>   (qemu) blockdev-snapshot-sync device=drive0 snapshot-file=sn1.qcow2
>          mode=absolute-paths format=qcow2
> 
> After blockdev-snapshot-sync completes the SCSI disk will be
> unresponsive.  This leads to request timeouts inside the guest.
> 
> Reported-by: Qianqian Zhu <address@hidden>
> Suggested-by: Fam Zheng <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> v3:
>  * s/dec_fetch/fetch_dec/ [Fam]
> ---
>  include/block/aio.h | 10 ++++++++--
>  1 file changed, 8 insertions(+), 2 deletions(-)

Thanks, applied to my block tree:
https://github.com/stefanha/qemu/commits/block

Stefan

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