linux-kernel - Re: Question on vhost_has

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Message-ID: <20140305171149.GA27966@redhat.com>
Date:	Wed, 5 Mar 2014 19:11:49 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question on vhost_has_feature()

On Tue, Mar 04, 2014 at 05:01:51PM -0800, Paul E. McKenney wrote:
> On Tue, Mar 04, 2014 at 09:56:42AM +0200, Michael S. Tsirkin wrote:
> > On Mon, Mar 03, 2014 at 11:44:23AM -0800, Paul E. McKenney wrote:
> > > Hello, Michael,
> > > 
> > > Just curious about the purpose of the rcu_dereference_index_check() in
> > > vhost_has_feature().  I don't see how it fits in.  The closest thing
> > > I see if the use in handle_rx(), where it selects vq->log or NULL.  But
> > > in that case, I would expect the usual RCU usage pattern to wrap an
> > > rcu_dereference() around the vq->log.
> > > 
> > > Enlightenment?
> > > 
> > > 							Thanx, Paul
> > 
> > Hi Paul,
> > 
> > Yes, it's weird.  As you say the use is in handle_rx.
> > The way it's supposed to work is that readers take vq mutex, and
> > writers change the value and then take and release
> > vq mutex.
> > We did it like this because there are thinkably multiple vqs.
> > 
> > I tried to document it in vhost.h :
> >         /* Readers use RCU to access memory table pointer
> >          * log base pointer and features.
> >          * Writers use mutex below.*/
> > 
> > If this is a problem, it's possible to restructure the code to avoid
> > this pattern for vhost_has_feature, pls let me know.
> 
> I am not yet sure whether or not it is a problem, it just looked a bit
> strange.  ;-)
> 
> 							Thanx, Paul
> 
> > Thanks a lot for paying attention!
> > 
> > 
> > -- 
> > MST
> > 

Or if you see a better way to document the use of rcu here
than the comment above, I'll gladly do it too.



-- 
MST
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