Re: [PATCH v7 2/3] tcm_vhost: Add helper to check if endpoint is setup

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.com>
> > > ---
> > >  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.


>    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.

> > >  static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> > >  {
> > >  	return 1;
> > > -- 
> > > 1.8.1.4
> 
> -- 
> Asias