linux-kernel - Re: Question on vhost_has

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Message-ID: <20140305010151.GA3334@linux.vnet.ibm.com>
Date:	Tue, 4 Mar 2014 17:01:51 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: Question on vhost_has_feature()

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
> 

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