Re: [Xen-devel] [PATCH v4 02/13] xen/pvcalls: implement frontend disconnect

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

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.

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