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Re: [PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del |
Date: |
Wed, 27 May 2020 14:11:48 +0100 |
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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- [PATCH v2 0/7] RFC/WIP: Fix scsi devices plug/unplug races w.r.t virtio-scsi iothread, Maxim Levitsky, 2020/05/11
- [PATCH v2 1/7] scsi/scsi_bus: switch search direction in scsi_device_find, Maxim Levitsky, 2020/05/11
- [PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del, Maxim Levitsky, 2020/05/11
- Re: [PATCH v2 2/7] Implement drain_call_rcu and use it in hmp_device_del,
Stefan Hajnoczi <=
- [PATCH v2 3/7] device-core: use RCU for list of childs of a bus, Maxim Levitsky, 2020/05/11
- [PATCH v2 4/7] device-core: use atomic_set on .realized property, Maxim Levitsky, 2020/05/11
- [PATCH v2 5/7] virtio-scsi: don't touch scsi devices that are not yet realized or about to be un-realized, Maxim Levitsky, 2020/05/11
- [PATCH v2 6/7] scsi: Add scsi_device_get, Maxim Levitsky, 2020/05/11
- [PATCH v2 7/7] virtio-scsi: use scsi_device_get, Maxim Levitsky, 2020/05/11