[libvirt] [PATCH v3 4/5] qemu_hotplug: Fix a rare race condition when detaching a device twice

[Peter Krempa]

On Fri, Mar 15, 2019 at 11:23:54 +0100, Michal Privoznik wrote:
> https://bugzilla.redhat.com/show_bug.cgi?id=1623389
> 
> If a device is detached twice from the same domain the following
> race condition may happen:
> 
> 1) The first DetachDevice() call will issue "device_del" on qemu
> monitor, but since the DEVICE_DELETED event did not arrive in
> time, the API ends claiming "Device detach request sent
> successfully".
> 
> 2) The second DetachDevice() therefore still find the device in
> the domain and thus proceeds to detaching it again. It calls
> EnterMonitor() and qemuMonitorSend() trying to issue "device_del"
> command again. This gets both domain lock and monitor lock
> released.
> 
> 3) At this point, qemu sends us the DEVICE_DELETED event which is
> going to be handled by the event loop which ends up calling
> qemuDomainSignalDeviceRemoval() to determine who is going to
> remove the device from domain definition. Whether it is the
> caller that marked the device for removal or whether it is going
> to be the event processing thread.
> 
> 4) Because the device was marked for removal,
> qemuDomainSignalDeviceRemoval() returns true, which means the
> event is to be processed by the thread that has marked the device
> for removal (and is currently still trying to issue "device_del"
> command)
> 
> 5) The thread finally issues the "device_del" command, which
> fails (obviously) and therefore it calls
> qemuDomainResetDeviceRemoval() to reset the device marking and
> quits immediately after, NOT removing any device from the domain
> definition.
> 
> At this point, the device is still present in the domain
> definition but doesn't exist in qemu anymore. Worse, there is no
> way to remove it from the domain definition.
> 
> Solution is to note down that we've seen the event and if the
> second "device_del" fails, not take it as a failure but carry on
> with the usual execution.
> 
> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
> ---
>  src/qemu/qemu_domain.h  |  1 +
>  src/qemu/qemu_hotplug.c | 32 ++++++++++++++++++++++++++++++--
>  2 files changed, 31 insertions(+), 2 deletions(-)

ACK
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