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Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handle
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler |
Date: |
Tue, 11 Sep 2018 17:30:41 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.9.1 |
On 11/09/2018 16:12, Fam Zheng wrote:
> On Tue, 09/11 13:32, Paolo Bonzini wrote:
>> 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?
>
> What do you mean by 'aio_wait_bh path'? Here is aio_wait_bh_oneshot:
Sorry, I meant the "atomic_inc(&wait_->num_waiters);" path. But if this
backtrace is obtained without dataplane, that's the answer I was seeking.
> void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque)
> {
> AioWaitBHData data = {
> .cb = cb,
> .opaque = opaque,
> };
>
> assert(qemu_get_current_aio_context() == qemu_get_aio_context());
>
> aio_bh_schedule_oneshot(ctx, aio_wait_bh, &data);
> AIO_WAIT_WHILE(&data.wait, ctx, !data.done);
> }
>
> ctx is qemu_aio_context here, so there's no interaction with IOThread.
In this case, it should be okay to have the reentrancy, what is the bug
that this patch is fixing?
Thanks,
Paolo
- [Qemu-block] [PATCH 0/2] virtio-scsi: Fix QEMU hang with vIOMMU and ATS, Fam Zheng, 2018/09/10
- [Qemu-block] [PATCH 1/2] virtio: Return true from virtio_queue_empty if broken, Fam Zheng, 2018/09/10
- [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Fam Zheng, 2018/09/10
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Paolo Bonzini, 2018/09/11
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Fam Zheng, 2018/09/11
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler,
Paolo Bonzini <=
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Fam Zheng, 2018/09/11
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Paolo Bonzini, 2018/09/12
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Fam Zheng, 2018/09/12
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Paolo Bonzini, 2018/09/12
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Fam Zheng, 2018/09/13
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Paolo Bonzini, 2018/09/13
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Paolo Bonzini, 2018/09/13
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Alex Williamson, 2018/09/13
- Re: [Qemu-block] [PATCH 2/2] virtio-scsi/virtio-blk: Disable poll handlers when stopping vq handler, Peter Xu, 2018/09/13