Re: [Qemu-devel] [PATCH] aio: add missing aio_notify() to aio_enable_external()

[Paolo Bonzini <pbonzini@redhat.com>]

On 04/05/2017 12:23, 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.

I agree this is the minimal fix and is the right thing to do.  The
bdrv_drained_begin/end device callbacks would also make it possible to
remove disable/enable external altogether, but that's more invasive.

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: qemu-stable@nongnu.org

> Reported-by: Qianqian Zhu <qizhu@redhat.com>
> Suggested-by: Fam Zheng <famz@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  include/block/aio.h | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/include/block/aio.h b/include/block/aio.h
> index 406e323..5294b04 100644
> --- a/include/block/aio.h
> +++ b/include/block/aio.h
> @@ -456,6 +456,7 @@ static inline void aio_enable_external(AioContext *ctx)
>  {
>      assert(ctx->external_disable_cnt > 0);
>      atomic_dec(&ctx->external_disable_cnt);
> +    aio_notify(ctx);
>  }
>  
>  /**
>