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Date: Thu, 2 Jul 2015 14:54:10 -0400
From: Konrad Rzeszutek Wilk <konrad@darnok.org>
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In-Reply-To: <alpine.DEB.2.02.1507011502340.17378@kaball.uk.xensource.com>
Subject: Re: [Qemu-devel] [Xen-devel] [PATCH RFC 1 6/8] xen/pt: Make
	xen_pt_unregister_device idempotent
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To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: xen-devel@lists.xenproject.org, qemu-devel@nongnu.org, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

> > @@ -858,15 +863,20 @@ static void xen_pt_unregister_device(PCIDevice *d)
> >                             machine_irq, errno);
> >              }
> >          }
> > +        s->machine_irq = 0;
> >      }
> >  
> >      /* delete all emulated config registers */
> >      xen_pt_config_delete(s);
> >  
> > -    memory_listener_unregister(&s->memory_listener);
> > -    memory_listener_unregister(&s->io_listener);
> > -
> > -    xen_host_pci_device_put(&s->real_device);
> > +    if (s->listener_set) {
> > +        memory_listener_unregister(&s->memory_listener);
> > +        memory_listener_unregister(&s->io_listener);
> > +        s->listener_set = false;
> 
> If you call QTAILQ_INIT on memory_listener and io_listener, then you
> simply check on QTAILQ_EMPTY and remove listener_set.

No dice. For that to work they need to be QTAIL_HEAD while
they are of QTAILQ_ENTRY.

That is, the include/exec/memory.h:
struct MemoryListener {
 ...
	AddressSpace *address_space_filter;
	QTAILQ_ENTRY(MemoryListener) link;
}

And the QTAILQ_EMPTY checks for '>tqh_first' while the
QTAILQ_ENTRY only defines two pointers: tqe_next and tqe_prev.