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

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Date:	Tue, 6 Jan 2015 09:15:49 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Venkatesh Srinivas <venkateshs@...gle.com>
Cc:	Fam Zheng <famz@...hat.com>, 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>
Subject: Re: [PATCH] virtio-scsi: Fix the race condition in
 virtscsi_handle_event

On Tue, Jan 06, 2015 at 12:10:59AM +0200, Michael S. Tsirkin wrote:
> 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@...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?
> 
> Or use the system single-threaded wq.


I was sure we have one, but apparently not :(

Pls ignore the comment, sorry about the noise.

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