linux-kernel - Re: [PATCH v2 1/2] pvcalls-front: introduce a per sock

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Date:   Wed, 14 Feb 2018 10:13:20 -0800 (PST)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Juergen Gross <jgross@...e.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
        linux-kernel@...r.kernel.org,
        Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v2 1/2] pvcalls-front: introduce a per sock_mapping
 refcount

On Wed, 14 Feb 2018, Juergen Gross wrote:
> On 13/02/18 03:13, Stefano Stabellini wrote:
> > Introduce a per sock_mapping refcount, in addition to the existing
> > global refcount. Thanks to the sock_mapping refcount, we can safely wait
> > for it to be 1 in pvcalls_front_release before freeing an active socket,
> > instead of waiting for the global refcount to be 1.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > 
> > ---
> > Changes in v2:
> > - fix code style
> > - nicer checks in pvcalls_front_release
> > - fix check in pvcalls_enter_sock
> > ---
> >  drivers/xen/pvcalls-front.c | 193 +++++++++++++++++++-------------------------
> >  1 file changed, 81 insertions(+), 112 deletions(-)
> > 
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index 4c789e6..163bf8c 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -60,6 +60,7 @@ struct sock_mapping {
> >  	bool active_socket;
> >  	struct list_head list;
> >  	struct socket *sock;
> > +	atomic_t refcount;
> >  	union {
> >  		struct {
> >  			int irq;
> > @@ -93,6 +94,34 @@ struct sock_mapping {
> >  	};
> >  };
> >  
> > +static inline struct sock_mapping *pvcalls_enter_sock(struct socket *sock)
> > +{
> > +	struct sock_mapping *map = NULL;
> 
> Pointless initializer.

I'll fix


> > +
> > +	if (!pvcalls_front_dev ||
> > +		dev_get_drvdata(&pvcalls_front_dev->dev) == NULL)
> > +		return ERR_PTR(-ENOTCONN);
> > +
> > +	pvcalls_enter();
> > +	map = (struct sock_mapping *)sock->sk->sk_send_head;
> > +	if (map == NULL) {
> > +		pvcalls_exit()
> > +		return ERR_PTR(-ENOTSOCK);
> > +	}
> 
> Sorry for noticing this only now: any reason you don't call
> pvcalls_enter() only here instead? This would remove the need of
> calling pvcalls_exit() if map == NULL.
> 
> I can't see pvcalls_enter() protecting sock->sk->sk_send_head in any
> way.

You are right. I'll move it down a couple of lines.


> > +
> > +	atomic_inc(&map->refcount);
> > +	return map;
> > +}
> > +
> > +static inline void pvcalls_exit_sock(struct socket *sock)
> > +{
> > +	struct sock_mapping *map = NULL;
> 
> Pointless initializer again.

I'll fix