Re: [PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del

[Stefan Hajnoczi]

On Mon, May 11, 2020 at 07:09:46PM +0300, Maxim Levitsky wrote:
>  /* The operands of the minus operator must have the same type,
>   * which must be the one that we specify in the cast.
> diff --git a/qdev-monitor.c b/qdev-monitor.c
> index 56cee1483f..70877840a2 100644
> --- a/qdev-monitor.c
> +++ b/qdev-monitor.c
> @@ -812,6 +812,8 @@ void qmp_device_add(QDict *qdict, QObject **ret_data, 
> Error **errp)
>          return;
>      }
>      dev = qdev_device_add(opts, &local_err);
> +    drain_call_rcu();

Please include comments explaining what each drain waits for. Without
comments we'll quickly lose track of why drain_call_rcu() calls are
necessary (similar to documenting memory barrier or refcount inc/dec
pairing).

> diff --git a/util/rcu.c b/util/rcu.c
> index 60a37f72c3..e8b1c4d6c5 100644
> --- a/util/rcu.c
> +++ b/util/rcu.c
> @@ -293,6 +293,39 @@ void call_rcu1(struct rcu_head *node, void 
> (*func)(struct rcu_head *node))
>      qemu_event_set(&rcu_call_ready_event);
>  }
>  
> +
> +struct rcu_drain {
> +    struct rcu_head rcu;
> +    QemuEvent drain_complete_event;
> +};
> +
> +static void drain_rcu_callback(struct rcu_head *node)
> +{
> +    struct rcu_drain *event = (struct rcu_drain *)node;
> +    qemu_event_set(&event->drain_complete_event);

A comment would be nice explaining that callbacks are invoked in
sequence so we're sure that all previously scheduled callbacks have
completed when drain_rcu_callback() is invoked.

> +}
> +
> +void drain_call_rcu(void)

Please document that the main loop mutex is dropped if it's held. This
will prevent surprises and allow callers to think about thread-safety
across this call.

Aside from the comment requests:
Reviewed-by: Stefan Hajnoczi <address@hidden>

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