linux-kernel - Re: [PATCH v4 02/13] xen/pvcalls: implement frontend disconnect

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Date:   Fri, 6 Oct 2017 10:51:06 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
cc:     Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xen.org, linux-kernel@...r.kernel.org,
        jgross@...e.com, Stefano Stabellini <stefano@...reto.com>
Subject: Re: [PATCH v4 02/13] xen/pvcalls: implement frontend disconnect

On Wed, 20 Sep 2017, Boris Ostrovsky wrote:
> > +
> > +struct pvcalls_bedata {
> > +	struct xen_pvcalls_front_ring ring;
> > +	grant_ref_t ref;
> > +	int irq;
> > +
> > +	struct list_head socket_mappings;
> > +	struct list_head socketpass_mappings;
> > +	spinlock_t socket_lock;
> > +
> > +	wait_queue_head_t inflight_req;
> > +	struct xen_pvcalls_response rsp[PVCALLS_NR_REQ_PER_RING];
> > +};
> > +static struct xenbus_device *pvcalls_front_dev;
> > +static atomic_t pvcalls_refcount;
> 
> Should the refcount be per back/frontend?

Yes it is, but only one back/frontend connection is supported by the
frontend. I can add a comment.


> > +
> > +/* first increment refcount, then proceed */
> > +#define pvcalls_enter {                     \
> > +	atomic_inc(&pvcalls_refcount);      \
> > +	smp_mb();                           \
> > +}
> > +
> > +/* first complete other operations, then decrement refcount */
> > +#define pvcalls_exit {                      \
> > +	smp_mb();                           \
> > +	atomic_dec(&pvcalls_refcount);      \
> > +}
> 
> I think atomic increment/decrement imply a barrier.

You are right. From Documentation/core-api/atomic_ops.rst:

One very important aspect of these two routines is that they DO NOT
require any explicit memory barriers.