linux-kernel - Re: Question on vhost_has

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Message-ID: <20140304075642.GA8031@redhat.com>
Date:	Tue, 4 Mar 2014 09:56:42 +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 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.

Thanks a lot for paying attention!

-- 
MST
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