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:
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. This is
the full backtrace:
https://paste.ubuntu.com/p/Dm7zGzmmRG/
Fam
