Re: [PATCH v4] virtio-scsi: Fix the race condition in virtscsi_handle_event

[Paolo Bonzini <pbonzini@redhat.com>]

On 08/01/2015 03:57, Fam Zheng wrote:
> There is a race condition in virtscsi_handle_event, when many device
> hotplug/unplug events flush in quickly.
> 
> The scsi_remove_device in virtscsi_handle_transport_reset may trigger
> the BUG_ON in scsi_target_reap, because the state is altered behind it,
> probably by scsi_scan_host of another event. I'm able to reproduce it by
> repeatedly plugging and unplugging a scsi disk with the same lun number.
> 
> To fix this, a single thread workqueue (local to the module) is added,
> which makes the scan work serialized. With this change, the panic goes
> away.
> 
> Signed-off-by: Fam Zheng <famz@redhat.com>

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

> ---
> 
> v4: Addressing MST's comments:
>     Use ordered workqueue, with WQ_FREEZABLE and WQ_MEM_RECLAIM flags.
>     Coding style fixes.
> 
> v3: Fix spacing and destroy order. (MST)
> ---
>  drivers/scsi/virtio_scsi.c | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index c52bb5d..0db63b5 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -120,6 +120,7 @@ struct virtio_scsi {
>  
>  static struct kmem_cache *virtscsi_cmd_cache;
>  static mempool_t *virtscsi_cmd_pool;
> +static struct workqueue_struct *virtscsi_scan_wq;
>  
>  static inline struct Scsi_Host *virtio_scsi_host(struct virtio_device *vdev)
>  {
> @@ -404,7 +405,7 @@ static void virtscsi_complete_event(struct virtio_scsi *vscsi, void *buf)
>  	struct virtio_scsi_event_node *event_node = buf;
>  
>  	if (!vscsi->stop_events)
> -		queue_work(system_freezable_wq, &event_node->work);
> +		queue_work(virtscsi_scan_wq, &event_node->work);
>  }
>  
>  static void virtscsi_event_done(struct virtqueue *vq)
> @@ -1119,6 +1120,14 @@ static int __init init(void)
>  		pr_err("mempool_create() for virtscsi_cmd_pool failed\n");
>  		goto error;
>  	}
> +
> +	virtscsi_scan_wq =
> +		alloc_ordered_workqueue("virtscsi-scan", WQ_FREEZABLE | WQ_MEM_RECLAIM);
> +	if (!virtscsi_scan_wq) {
> +		pr_err("create_singlethread_workqueue() for virtscsi_scan_wq failed\n");
> +		goto error;
> +	}
> +
>  	ret = register_virtio_driver(&virtio_scsi_driver);
>  	if (ret < 0)
>  		goto error;
> @@ -1126,6 +1135,8 @@ static int __init init(void)
>  	return 0;
>  
>  error:
> +	if (virtscsi_scan_wq)
> +		destroy_workqueue(virtscsi_scan_wq);
>  	if (virtscsi_cmd_pool) {
>  		mempool_destroy(virtscsi_cmd_pool);
>  		virtscsi_cmd_pool = NULL;
> @@ -1140,6 +1151,7 @@ error:
>  static void __exit fini(void)
>  {
>  	unregister_virtio_driver(&virtio_scsi_driver);
> +	destroy_workqueue(virtscsi_scan_wq);
>  	mempool_destroy(virtscsi_cmd_pool);
>  	kmem_cache_destroy(virtscsi_cmd_cache);
>  }
>