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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Tue, 11 Sep 2018 13:32:13 +0200
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Subject: Re: [Qemu-devel] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll
 handlers when stopping vq handler
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To: Fam Zheng <famz@redhat.com>, qemu-devel@nongnu.org
Cc: qemu-block@nongnu.org, "Michael S. Tsirkin" <mst@redhat.com>, peterx@redhat.com, Stefan Hajnoczi <stefanha@redhat.com>

On 10/09/2018 16:56, Fam Zheng wrote:
> We have this unwanted call stack:
> 
>   > ...
>   > #13 0x00005586602b7793 in virtio_scsi_handle_cmd_vq
>   > #14 0x00005586602b8d66 in virtio_scsi_data_plane_handle_cmd
>   > #15 0x00005586602ddab7 in virtio_queue_notify_aio_vq
>   > #16 0x00005586602dfc9f in virtio_queue_host_notifier_aio_poll
>   > #17 0x00005586607885da in run_poll_handlers_once
>   > #18 0x000055866078880e in try_poll_mode
>   > #19 0x00005586607888eb in aio_poll
>   > #20 0x0000558660784561 in aio_wait_bh_oneshot
>   > #21 0x00005586602b9582 in virtio_scsi_dataplane_stop
>   > #22 0x00005586605a7110 in virtio_bus_stop_ioeventfd
>   > #23 0x00005586605a9426 in virtio_pci_stop_ioeventfd
>   > #24 0x00005586605ab808 in virtio_pci_common_write
>   > #25 0x0000558660242396 in memory_region_write_accessor
>   > #26 0x00005586602425ab in access_with_adjusted_size
>   > #27 0x0000558660245281 in memory_region_dispatch_write
>   > #28 0x00005586601e008e in flatview_write_continue
>   > #29 0x00005586601e01d8 in flatview_write
>   > #30 0x00005586601e04de in address_space_write
>   > #31 0x00005586601e052f in address_space_rw
>   > #32 0x00005586602607f2 in kvm_cpu_exec
>   > #33 0x0000558660227148 in qemu_kvm_cpu_thread_fn
>   > #34 0x000055866078bde7 in qemu_thread_start
>   > #35 0x00007f5784906594 in start_thread
>   > #36 0x00007f5784639e6f in clone
> 
> Avoid it with the aio_disable_external/aio_enable_external pair, so that
> no vq poll handlers can be called in aio_wait_bh_oneshot.

I don't understand.  We are in the vCPU thread, so not in the
AioContext's home thread.  Why is aio_wait_bh_oneshot polling rather
than going through the aio_wait_bh path?

Thanks,

Paolo