[PATCH v2 10/13] qemu: avoid deadlock in qemuDomainObjStopWorker

[Daniel P. Berrangé]

On Thu, Jul 23, 2020 at 01:14:10PM +0300, Nikolay Shirokovskiy wrote:
> We are dropping the only reference here so that the event loop thread
> is going to be exited synchronously. In order to avoid deadlocks we
> need to unlock the VM so that any handler being called can finish
> execution and thus even loop thread be finished too.
> 
> Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy at virtuozzo.com>
> ---
>  src/qemu/qemu_domain.c | 18 ++++++++++++++----
>  1 file changed, 14 insertions(+), 4 deletions(-)
> 
> diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> index 5b22eb2..82b3d11 100644
> --- a/src/qemu/qemu_domain.c
> +++ b/src/qemu/qemu_domain.c
> @@ -1637,11 +1637,21 @@ void
>  qemuDomainObjStopWorker(virDomainObjPtr dom)
>  {
>      qemuDomainObjPrivatePtr priv = dom->privateData;
> +    virEventThread *eventThread;
>  
> -    if (priv->eventThread) {
> -        g_object_unref(priv->eventThread);
> -        priv->eventThread = NULL;
> -    }
> +    if (!priv->eventThread)
> +        return;
> +
> +    /*
> +     * We are dropping the only reference here so that the event loop thread
> +     * is going to be exited synchronously. In order to avoid deadlocks we
> +     * need to unlock the VM so that any handler being called can finish
> +     * execution and thus even loop thread be finished too.
> +     */
> +    eventThread = g_steal_pointer(&priv->eventThread);
> +    virObjectUnlock(dom);
> +    g_object_unref(eventThread);
> +    virObjectLock(dom);

Hmm, I'm really not at all comfortable with this. I don't have confidence
that qemuProcessStop() safe if we drpo & reacquire the lock half way
through its cleanup process.  The 'virDomainObj' is in a semi-clean
state, 1/2 running 1/2 shutoff.

This is the key reason I think I didn't do the synchronous thread join
in the event loop.

Regards,
Daniel
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