On Thu, Mar 07, 2013 at 08:26:20AM +0800, Asias He wrote:
> On Wed, Mar 06, 2013 at 10:21:09AM +0100, Stefan Hajnoczi wrote:
> > On Wed, Mar 06, 2013 at 02:16:30PM +0800, Asias He wrote:
> > > +static struct tcm_vhost_evt *tcm_vhost_allocate_evt(struct vhost_scsi *vs,
> > > + u32 event, u32 reason)
> > > +{
> > > + struct tcm_vhost_evt *evt;
> > > +
> > > + if (atomic_read(&vs->vs_events_nr) > VHOST_SCSI_MAX_EVENT)
> > > + return NULL;
> > > +
> > > + evt = kzalloc(sizeof(*evt), GFP_KERNEL);
> > > +
> > > + if (evt) {
> > > + atomic_inc(&vs->vs_events_nr);
> >
> > This looks suspicious: checking vs_events_nr > VHOST_SCSI_MAX_EVENT
> > first and then incrementing later isn't atomic!
>
> This does not matter. (1) and (2) are okay. In case (3), the other side
> can only decrease the number of event, the limit will not be exceeded.
>
> (1)
> atomic_dec()
> atomic_read()
> atomic_inc()
> (2)
> atomic_read()
> atomic_inc()
> atomic_dec()
>
> (3)
> atomic_read()
> atomic_dec()
> atomic_inc()
The cases you listed are fine but I'm actually concerned about
tcm_vhost_allocate_evt() racing with itself. There are 3 callers and
I'm not sure which lock prevents them from executing at the same time.
> > > +static int tcm_vhost_hotunplug(struct tcm_vhost_tpg *tpg, struct se_lun *lun)
> > > +{
> > > + struct vhost_scsi *vs = tpg->vhost_scsi;
> > > +
> > > + mutex_lock(&tpg->tv_tpg_mutex);
> > > + vs = tpg->vhost_scsi;
> > > + mutex_unlock(&tpg->tv_tpg_mutex);
> > > + if (!vs)
> > > + return -EOPNOTSUPP;
> > > +
> > > + if (!tcm_vhost_check_feature(vs, VIRTIO_SCSI_F_HOTPLUG))
> > > + return -EOPNOTSUPP;
> > > +
> > > + return tcm_vhost_send_evt(vs, tpg, lun,
> > > + VIRTIO_SCSI_T_TRANSPORT_RESET,
> > > + VIRTIO_SCSI_EVT_RESET_REMOVED);
> > > +}
> >
> > tcm_vhost_hotplug() and tcm_vhost_hotunplug() are the same function
> > except for VIRTIO_SCSI_EVT_RESET_RESCAN vs
> > VIRTIO_SCSI_EVT_RESET_REMOVED. That can be passed in as an argument and
> > the code duplication can be eliminated.
>
> I thought about this also. We can have a tcm_vhost_do_hotplug() helper.
>
> tcm_vhost_do_hotplug(tpg, lun, plug)
>
> tcm_vhost_hotplug() {
> tcm_vhost_do_hotplug(tpg, lun, true)
> }
>
> tcm_vhost_hotunplug() {
> tcm_vhost_do_hotplug(tpg, lun, false)
> }
>
> The reason I did not do that is I do not like the true/false argument
> but anyway this could remove duplication. I will do it.
true/false makes the calling code hard to read, I suggest passing in
VIRTIO_SCSI_EVT_RESET_RESCAN or VIRTIO_SCSI_EVT_RESET_REMOVED as the
argument.
Stefan
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