On Mon, Jan 05, 2015 at 11:48:47AM -0800, Venkatesh Srinivas wrote:
> On Sun, Jan 4, 2015 at 10:04 PM, Fam Zheng <famz@xxxxxxxxxx> 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 make is safe, the mutex added in struct virtio_scsi is held in
> virtscsi_handle_event, so that all the events are processed in a
> synchronized way. With this lock, the panic goes away.
>
> Signed-off-by: Fam Zheng <famz@xxxxxxxxxx>
> ---
> drivers/scsi/virtio_scsi.c | 6 ++++++
> 1 file changed, 6 insertions(+)
>
> diff --git a/drivers/scsi/virtio_scsi.c b/drivers/scsi/virtio_scsi.c
> index c52bb5d..7f194d4 100644
> --- a/drivers/scsi/virtio_scsi.c
> +++ b/drivers/scsi/virtio_scsi.c
> @@ -110,6 +110,9 @@ struct virtio_scsi {
> /* CPU hotplug notifier */
> struct notifier_block nb;
>
> + /* Protect the hotplug/unplug event handling */
> + struct mutex scan_lock;
> +
> /* Protected by event_vq lock */
> bool stop_events;
>
> @@ -377,6 +380,7 @@ static void virtscsi_handle_event(struct work_struct
> *work)
> struct virtio_scsi *vscsi = event_node->vscsi;
> struct virtio_scsi_event *event = &event_node->event;
>
> + mutex_lock(&vscsi->scan_lock);
> if (event->event &
> cpu_to_virtio32(vscsi->vdev, VIRTIO_SCSI_T_EVENTS_MISSED)) {
> event->event &= ~cpu_to_virtio32(vscsi->vdev,
> @@ -397,6 +401,7 @@ static void virtscsi_handle_event(struct work_struct
> *work)
> pr_err("Unsupport virtio scsi event %x\n", event->event);
> }
> virtscsi_kick_event(vscsi, event_node);
> + mutex_unlock(&vscsi->scan_lock);
> }
>
> static void virtscsi_complete_event(struct virtio_scsi *vscsi, void *buf)
> @@ -894,6 +899,7 @@ static int virtscsi_init(struct virtio_device *vdev,
> const char **names;
> struct virtqueue **vqs;
>
> + mutex_init(&vscsi->scan_lock);
> num_vqs = vscsi->num_queues + VIRTIO_SCSI_VQ_BASE;
> vqs = kmalloc(num_vqs * sizeof(struct virtqueue *), GFP_KERNEL);
> callbacks = kmalloc(num_vqs * sizeof(vq_callback_t *), GFP_KERNEL);
> --
> 1.9.3
>
>
> Nice find.
>
> This fix does have the effect of serializing all event handling via scan_lock;
> perhaps you want to instead create a singlethreaded workqueue in virtio_scsi
> and queue handle_event there, rather than waiting on scan_lock on the system
> workqueue?
Or use the system single-threaded wq.
> Reviewed-by: Venkatesh Srinivas <venkateshs@xxxxxxxxxx>
>
> -- vs;
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