From mboxrd@z Thu Jan  1 00:00:00 1970
From: Asias He <asias@redhat.com>
Subject: Re: [PATCH 2/4] tcm_vhost: Introduce tcm_vhost_check_endpoint()
Date: Thu, 14 Mar 2013 10:14:02 +0800
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>, virtualization@lists.linux-foundation.org, target-devel@vger.kernel.org, Stefan Hajnoczi <stefanha@redhat.com>
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On Wed, Mar 13, 2013 at 09:00:43AM +0100, Paolo Bonzini wrote:
> Il 13/03/2013 04:02, Asias He ha scritto:
> > 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?
> 
> Yes, it's just what you have with tcm_vhost_check_endpoint inlined.

okay.

> > 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.
> 
> I mean (2).  You could use an rwlock to enable more concurrency.

-- 
Asias