Re: [PATCH 2/4] tcm_vhost: Introduce tcm_vhost_check_endpoint()

[Asias He <asias@redhat.com>]

On Tue, Mar 12, 2013 at 09:26:18AM +0100, Paolo Bonzini wrote:
> Il 12/03/2013 03:42, Asias He ha scritto:
> > This helper is useful to check if vs->vs_endpoint is setup by
> > vhost_scsi_set_endpoint()
> > 
> > Signed-off-by: Asias He <asias@redhat.com>
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  drivers/vhost/tcm_vhost.c | 12 ++++++++++++
> >  1 file changed, 12 insertions(+)
> > 
> > diff --git a/drivers/vhost/tcm_vhost.c b/drivers/vhost/tcm_vhost.c
> > index b3e50d7..29612bc 100644
> > --- a/drivers/vhost/tcm_vhost.c
> > +++ b/drivers/vhost/tcm_vhost.c
> > @@ -91,6 +91,18 @@ static int iov_num_pages(struct iovec *iov)
> >  	       ((unsigned long)iov->iov_base & PAGE_MASK)) >> PAGE_SHIFT;
> >  }
> >  
> > +static bool tcm_vhost_check_endpoint(struct vhost_scsi *vs)
> > +{
> > +	bool ret = false;
> > +
> > +	mutex_lock(&vs->dev.mutex);
> > +	if (vs->vs_endpoint)
> > +		ret = true;
> > +	mutex_unlock(&vs->dev.mutex);
> 
> The return value is invalid as soon as mutex_unlock is called, i.e.
> before tcm_vhost_check_endpoint returns.  Instead, check vs->vs_endpoint
> in the caller while the mutex is taken.

Do you mean 1) or 2)?

   1)
   vhost_scsi_handle_vq()
   {
   
      mutex_lock(&vs->dev.mutex);
      check vs->vs_endpoint
      mutex_unlock(&vs->dev.mutex);
   
      handle vq
   }
   
   2)
   vhost_scsi_handle_vq()
   {
   
      lock vs->dev.mutex
      check vs->vs_endpoint
      handle vq
      unlock vs->dev.mutex
   }

1) does not make any difference with the original
one right?

2) would be too heavy. This might not be a problem in current 1 thread
per vhost model. But if we want concurrent multiqueue, this will be
killing us.

Anyway, the current one is not good. Need to think.

> Paolo
> 
> > +	return ret;
> > +}
> > +
> >  static int tcm_vhost_check_true(struct se_portal_group *se_tpg)
> >  {
> >  	return 1;
> > 
> 

-- 
Asias