On Thu, Apr 18, 2013 at 10:38:23AM +0300, Michael S. Tsirkin wrote:
> On Thu, Apr 18, 2013 at 04:32:30PM +0800, Asias He wrote:
> > On Thu, Apr 18, 2013 at 10:09:53AM +0300, Michael S. Tsirkin wrote:
> > > On Thu, Apr 18, 2013 at 09:05:53AM +0800, Asias He wrote:
> > > > Signed-off-by: Asias He <asias@xxxxxxxxxx>
> > > > ---
> > > > drivers/vhost/tcm_vhost.c | 18 ++++++++++++++++++
> > > > 1 file changed, 18 insertions(+)
> > > >
> > > > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > > > index 88ebb79..8f05528 100644
> > > > --- a/drivers/vhost/tcm_vhost.c
> > > > +++ b/drivers/vhost/tcm_vhost.c
> > > > @@ -111,6 +111,24 @@ static bool tcm_vhost_check_feature(struct vhost_scsi *vs, int feature)
> > > > return ret;
> > > > }
> > > >
> > > > +static bool tcm_vhost_check_endpoint(struct vhost_virtqueue *vq)
> > > > +{
> > > > + bool ret = false;
> > > > +
> > > > + /*
> > > > + * We can handle the vq only after the endpoint is setup by calling the
> > > > + * VHOST_SCSI_SET_ENDPOINT ioctl.
> > > > + *
> > > > + * TODO: Check that we are running from vhost_worker which acts
> > > > + * as read-side critical section for vhost kind of RCU.
> > > > + * See the comments in struct vhost_virtqueue in drivers/vhost/vhost.h
> > > > + */
> > > > + if (rcu_dereference_check(vq->private_data, 1))
> > > > + ret = true;
> > > > +
> > > > + return ret;
> > > > +}
> > > > +
> > >
> > > You can't use RCU in this way. You need to actually
> > > derefence. Instead, just move this within vq mutex
> > > and do rcu_dereference_protected. document that this function
> > > requires vq mutex.
> >
> > Any difference with:
> >
> > vs_tpg = rcu_dereference_check(vq->private_data, 1);
>
> Yes, it's not the same. rcu_dereference_protected says
> "this is generally RCU but here I use it under lock".
> rcu_dereference_check can only be used if you
> really dereference later, and you don't.
So you want the helper to be this?
static bool tcm_vhost_check_endpoint()
{
struct tcm_vhost_tpg **vs_tpg;
vs_tpg = rcu_dereference_check(vq->private_data, 1);
if (vs_tpg)
return true;
else
return false
}
And what difference code the compiler will generate with this:
static bool tcm_vhost_check_endpoint()
{
if (rcu_dereference_check(vq->private_data, 1))
return true
else
return false
}
>
> > if (vs_tpg)
> > return ture
> > else
> > return false;
> >
> > Note, tcm_vhost_check_endpoint() is called in multiple places. Having a
> > helper is more convenient.
>
> I'm fine with the helper, but fix the implementation
> and fix all callers to have vq mutex when calling.
All the caller are in the vhost thread. Why need vq mutex to be taken?
> > > > static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> > > > {
> > > > return 1;
> > > > --
> > > > 1.8.1.4
> >
> > --
> > Asias
--
Asias
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