linux-kernel - Re: [PATCH v5 02/13] xen/pvcalls: implement frontend disconnect

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Date:   Mon, 23 Oct 2017 15:44:30 -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 v5 02/13] xen/pvcalls: implement frontend disconnect

On Tue, 17 Oct 2017, Boris Ostrovsky wrote:
> On 10/06/2017 08:30 PM, Stefano Stabellini wrote:
> > Introduce a data structure named pvcalls_bedata. It contains pointers to
> > the command ring, the event channel, a list of active sockets and a list
> > of passive sockets. Lists accesses are protected by a spin_lock.
> >
> > Introduce a waitqueue to allow waiting for a response on commands sent
> > to the backend.
> >
> > Introduce an array of struct xen_pvcalls_response to store commands
> > responses.
> >
> > pvcalls_refcount is used to keep count of the outstanding pvcalls users.
> > Only remove connections once the refcount is zero.
> >
> > Implement pvcalls frontend removal function. Go through the list of
> > active and passive sockets and free them all, one at a time.
> >
> > Signed-off-by: Stefano Stabellini <stefano@...reto.com>
> > CC: boris.ostrovsky@...cle.com
> > CC: jgross@...e.com
> > ---
> >  drivers/xen/pvcalls-front.c | 67 +++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 67 insertions(+)
> >
> > diff --git a/drivers/xen/pvcalls-front.c b/drivers/xen/pvcalls-front.c
> > index a8d38c2..d8b7a04 100644
> > --- a/drivers/xen/pvcalls-front.c
> > +++ b/drivers/xen/pvcalls-front.c
> > @@ -20,6 +20,46 @@
> >  #include <xen/xenbus.h>
> >  #include <xen/interface/io/pvcalls.h>
> >  
> > +#define PVCALLS_INVALID_ID UINT_MAX
> > +#define PVCALLS_RING_ORDER XENBUS_MAX_RING_GRANT_ORDER
> > +#define PVCALLS_NR_REQ_PER_RING __CONST_RING_SIZE(xen_pvcalls, XEN_PAGE_SIZE)
> > +
> > +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];
> 
> Did you mean _REQ_ or _RSP_ in the macro name?

For each request there is one response, so it doesn't make a difference.
But for clarity, I will rename.


> > +};
> > +/* Only one front/back connection supported. */
> > +static struct xenbus_device *pvcalls_front_dev;
> > +static atomic_t pvcalls_refcount;
> > +
> > +/* first increment refcount, then proceed */
> > +#define pvcalls_enter() {               \
> > +	atomic_inc(&pvcalls_refcount);      \
> > +}
> > +
> > +/* first complete other operations, then decrement refcount */
> > +#define pvcalls_exit() {                \
> > +	atomic_dec(&pvcalls_refcount);      \
> > +}
> > +
> > +static irqreturn_t pvcalls_front_event_handler(int irq, void *dev_id)
> > +{
> > +	return IRQ_HANDLED;
> > +}
> > +
> > +static void pvcalls_front_free_map(struct pvcalls_bedata *bedata,
> > +				   struct sock_mapping *map)
> > +{
> > +}
> > +
> >  static const struct xenbus_device_id pvcalls_front_ids[] = {
> >  	{ "pvcalls" },
> >  	{ "" }
> > @@ -27,6 +67,33 @@
> >  
> >  static int pvcalls_front_remove(struct xenbus_device *dev)
> >  {
> > +	struct pvcalls_bedata *bedata;
> > +	struct sock_mapping *map = NULL, *n;
> > +
> > +	bedata = dev_get_drvdata(&pvcalls_front_dev->dev);
> > +	dev_set_drvdata(&dev->dev, NULL);
> > +	pvcalls_front_dev = NULL;
> > +	if (bedata->irq >= 0)
> > +		unbind_from_irqhandler(bedata->irq, dev);
> > +
> > +	smp_mb();
> > +	while (atomic_read(&pvcalls_refcount) > 0)
> > +		cpu_relax();
> > +	list_for_each_entry_safe(map, n, &bedata->socket_mappings, list) {
> > +		pvcalls_front_free_map(bedata, map);
> > +		kfree(map);
> > +	}
> > +	list_for_each_entry_safe(map, n, &bedata->socketpass_mappings, list) {
> > +		spin_lock(&bedata->socket_lock);
> > +		list_del_init(&map->list);
> > +		spin_unlock(&bedata->socket_lock);
> > +		kfree(map);
> 
> Why do you re-init the entry if you are freeing it?

Fair enough, I'll just list_del.


> And do you really
> need the locks around it? This looks similar to the case we've discussed
> for other patches --- if we are concerned that someone may grab this
> entry then something must be wrong.
> 
> (Sorry, this must have been here in earlier versions but I only now
> noticed it.)

Yes, you are right, it is already protected by the global refcount, I'll
remove.


> > +	}
> > +	if (bedata->ref >= 0)
> > +		gnttab_end_foreign_access(bedata->ref, 0, 0);
> > +	kfree(bedata->ring.sring);
> > +	kfree(bedata);
> > +	xenbus_switch_state(dev, XenbusStateClosed);
> >  	return 0;
> >  }
> >  
>