linux-kernel - Re: [PATCH] virtio-scsi: Fix the race condition in virtscsi_handle

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Date:	Mon, 5 Jan 2015 11:51:43 -0800
From:	Venkatesh Srinivas <venkateshs@...gle.com>
To:	Fam Zheng <famz@...hat.com>
Cc:	linux-scsi@...r.kernel.org,
	"James E.J. Bottomley" <JBottomley@...allels.com>,
	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Christoph Hellwig <hch@....de>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH] virtio-scsi: Fix the race condition in virtscsi_handle_event

On Sun, Jan 4, 2015 at 10:04 PM, Fam Zheng <famz@...hat.com> 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@...hat.com>
> ---
>  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?

Reviewed-by: Venkatesh Srinivas <venkateshs@...gle.com>

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